diff --git a/cove/cove_360/fixtures/badfile_too_many_rows.xlsx b/cove/cove_360/fixtures/badfile_too_many_rows.xlsx
new file mode 100644
index 00000000..00806a60
Binary files /dev/null and b/cove/cove_360/fixtures/badfile_too_many_rows.xlsx differ
diff --git a/cove/cove_360/fixtures/badfile_too_many_rows_multiple_sheets.xlsx b/cove/cove_360/fixtures/badfile_too_many_rows_multiple_sheets.xlsx
new file mode 100644
index 00000000..b69d898d
Binary files /dev/null and b/cove/cove_360/fixtures/badfile_too_many_rows_multiple_sheets.xlsx differ
diff --git a/cove/cove_360/tests/test_browser.py b/cove/cove_360/tests/test_browser.py
index cdf37448..9b6506ea 100644
--- a/cove/cove_360/tests/test_browser.py
+++ b/cove/cove_360/tests/test_browser.py
@@ -150,31 +150,17 @@ def server_url(request, live_server):
         'bad currency',
     ], True),
     ('badfile_all_validation_errors_4_times.xlsx', [
-        'Description is missing but required (more info about this error)',
-        'id is missing but required within recipientOrganization (more info about this error)',
-        'Date is not in the correct format (more info about this error)',
-        'Amount Awarded is not a number. Check that the value is not null, and doesn’t contain any characters other than 0-9 and dot (.). Number values should not be in quotes.',
-        'Invalid \'uri\' found (more info about this error)',
-        'Invalid code found in Currency (more info about this error)',
-        '[] is too short. You must supply at least one value, or remove the item entirely (unless it’s required).',
-        # Context dates should be ISO formatted
-        '2019-06-01T00:00:00+00:00',
-        'bad date 1',
-        'bad date 2',
-        'bad date 3',
-        'bad date 4',
-        'This should be a number',
-        'This should be a uri 1',
-        'This should be a uri 2',
-        # 'This should be a uri 3',
-        'This should be a uri 5',
-        'This should be a uri 6',
-        # 'This should be a uri 7',
-        'bad currency 1',
-        'bad currency 2',
-        'bad currency 3',
-        'bad currency 4',
+        '',
     ], True),
+    ('badfile_too_many_rows.xlsx', [
+        'This XLSX workbook has a worksheet (grants) with 50001 rows '
+        'but the maximum number of rows supported by this tool is 50000'
+    ], False),
+    ('badfile_too_many_rows_multiple_sheets.xlsx', [
+        'This XLSX workbook has worksheets with a larger number of rows '
+        'than is supported by this tool (50000). Worksheets with too many rows: '
+        '\'grants\' (50001 rows), \'grants_2\' (50002 rows)'
+    ], False),
     ("dei_extension.xlsx", [
         "do not use the 360Giving Data Standard codelists correctly.",
     ], True),
diff --git a/cove/cove_360/views.py b/cove/cove_360/views.py
index 11ada8aa..c65a0b89 100644
--- a/cove/cove_360/views.py
+++ b/cove/cove_360/views.py
@@ -5,6 +5,7 @@
 import logging
 import re
 import os
+import zipfile
 from decimal import Decimal
 
 from cove.views import explore_data_context, cove_web_input_error
@@ -17,6 +18,8 @@
 from django.utils.translation import gettext_lazy as _
 from django.core.cache import cache
 
+import openpyxl
+
 from libcove.config import LibCoveConfig
 from libcove.lib.converters import convert_spreadsheet, convert_json
 from libcove.lib.exceptions import CoveInputDataError
@@ -145,6 +148,40 @@ def explore_360(request, pk, template='cove_360/explore.html'):
                                     lib_cove_config=lib_cove_config))
 
     else:
+        if file_type == "xlsx":
+
+            # Check for an excessive number of rows before passing to flattentool.
+            excessive_sheets = {}
+            try:
+                workbook = openpyxl.reader.excel.load_workbook(file_name, read_only=True)
+                excessive_sheets = {
+                    sheetname: workbook[sheetname].max_row
+                    for sheetname in workbook.sheetnames
+                    if workbook[sheetname].max_row > settings.MAX_XLSX_ROWS
+                }
+
+            except (zipfile.BadZipFile, openpyxl.utils.exceptions.InvalidFileException):
+                # Exceptions associated with invalid spreadsheets are passed through for cove to handle.
+                pass
+
+            if len(excessive_sheets) == 1:
+                sheetname = next(iter(excessive_sheets))
+                wrapped_err = f"This XLSX workbook has a worksheet ({sheetname}) with " \
+                              f"{excessive_sheets[sheetname]} rows " \
+                              f"but the maximum number of rows supported by this tool is {settings.MAX_XLSX_ROWS}"
+                raise CoveInputDataError(wrapped_err=wrapped_err)
+            elif len(excessive_sheets) > 1:
+                wrapped_err = "This XLSX workbook has worksheets with a larger number of rows " \
+                              f"than is supported by this tool ({settings.MAX_XLSX_ROWS}). " \
+                              "Worksheets with too many rows: "
+                wrapped_err += ", ".join(
+                    [
+                        f"'{sheetname}' ({num_rows} rows)"
+                        for sheetname, num_rows in excessive_sheets.items()
+                    ]
+                )
+                raise CoveInputDataError(wrapped_err=wrapped_err)
+
         # Convert spreadsheet to json
         context.update(convert_spreadsheet(upload_dir, upload_url, file_name, file_type, lib_cove_config, schema_360.schema_file,
                                            schema_360.pkg_schema_file))
diff --git a/cove/cove_project/settings.py b/cove/cove_project/settings.py
index 0057c34f..60c079b0 100644
--- a/cove/cove_project/settings.py
+++ b/cove/cove_project/settings.py
@@ -12,6 +12,7 @@
     MEDIA_URL=(str, "/media/"),
     STATIC_ROOT=(str, os.path.join(BASE_DIR, "static")),
     STATIC_URL=(str, "/static/"),
+    MAX_XLSX_ROWS=(int, 50000),
 )
 
 # We use the setting to choose whether to show the section about Sentry in the
@@ -35,6 +36,7 @@
 SECRET_KEY = settings.SECRET_KEY
 DEBUG = settings.DEBUG
 ALLOWED_HOSTS = settings.ALLOWED_HOSTS
+MAX_XLSX_ROWS = env("MAX_XLSX_ROWS")
 
 MIDDLEWARE = (
     'django.contrib.sessions.middleware.SessionMiddleware',