diff --git a/README.md b/README.md
index d15b36c151ff7..08c28a8c8a154 100644
--- a/README.md
+++ b/README.md
@@ -61,7 +61,7 @@ Here are just a few of the things that pandas does well:
   - Intuitive [**merging**][merging] and [**joining**][joining] data
     sets
   - Flexible [**reshaping**][reshape] and [**pivoting**][pivot-table] of
-    data sets
+    data sets.
   - [**Hierarchical**][mi] labeling of axes (possible to have multiple
     labels per tick)
   - Robust I/O tools for loading data from [**flat files**][flat-files]
diff --git a/pandas/tests/groupby/aggregate/test_large_int_sum.py b/pandas/tests/groupby/aggregate/test_large_int_sum.py
new file mode 100644
index 0000000000000..791a96361a404
--- /dev/null
+++ b/pandas/tests/groupby/aggregate/test_large_int_sum.py
@@ -0,0 +1,135 @@
+import numpy as np
+import pandas as pd
+import pytest
+
+
+def test_groupby_sum_single_group_large_int64_matches_df_sum():
+    df = pd.DataFrame({"gb": ["A", "A"], "val": pd.Series([14, 2**60], dtype="int64")})
+    got = df.groupby("gb")["val"].sum().iloc[0]
+    exp = df["val"].sum()
+    assert got == exp
+    assert df["val"].dtype == "int64"
+    assert df.groupby("gb")["val"].sum().dtype == "int64"
+
+
+def test_groupby_sum_multi_groups_matches_series_sum_int64():
+    vals = pd.Series([2**60, 14, 2**60 + 3, 7], dtype="int64")
+    gb = pd.Series(["A", "A", "B", "B"])
+    df = pd.DataFrame({"gb": gb, "val": vals})
+
+    got = df.groupby("gb")["val"].sum()
+    exp = pd.Series(
+        {"A": vals.iloc[:2].sum(), "B": vals.iloc[2:].sum()},
+        dtype="int64",
+    )
+    exp.index.name = "gb"
+    exp.name = "val"       # <- this aligns the Series name with got
+
+    pd.testing.assert_series_equal(got, exp)
+
+
+@pytest.mark.parametrize(
+    "dtype, big, small",
+    [
+        ("int64", 2**60, 123),
+        ("uint64", np.uint64(2**60), np.uint64(123)),
+    ],
+)
+def test_groupby_sum_preserves_dtype_no_float_cast(dtype, big, small):
+    df = pd.DataFrame(
+        {"gb": ["A", "A", "B"], "val": pd.Series([big, small, big], dtype=dtype)}
+    )
+    out = df.groupby("gb")["val"].sum()
+    assert out.dtype.name == dtype
+    assert out.loc["A"] == pd.Series([big, small], dtype=dtype).sum()
+    assert out.loc["B"] == big
+
+
+def test_groupby_sum_nullable_uint64_min_count_behavior():
+    s = pd.Series([pd.NA, np.uint64(2**60)], dtype="UInt64")
+    df = pd.DataFrame({"gb": ["A", "A"], "val": s})
+
+    out_na = df.groupby("gb")["val"].sum(min_count=2)
+    assert out_na.dtype.name == "UInt64"
+    assert out_na.iloc[0] is pd.NA
+
+    out_ok = df.groupby("gb")["val"].sum(min_count=1)
+    assert out_ok.dtype.name == "UInt64"
+    assert out_ok.iloc[0] == np.uint64(2**60)
+
+
+def test_groupby_sum_nullable_all_na_respects_min_count():
+    s = pd.Series([pd.NA, pd.NA], dtype="Int64")
+    df = pd.DataFrame({"gb": ["A", "A"], "val": s})
+    out = df.groupby("gb")["val"].sum(min_count=1)
+    assert out.dtype.name == "Int64"
+    assert out.iloc[0] is pd.NA
+
+
+def test_groupby_sum_dataframe_multiple_integer_columns_preserve_dtypes():
+    # int64 + uint64 columns; ensure values and dtypes preserved
+    df = pd.DataFrame(
+        {
+            "gb": ["A", "A", "B"],
+            "i64": pd.Series([2**60, 5, 7], dtype="int64"),
+            "u64": pd.Series(
+                [np.uint64(10), np.uint64(2**54), np.uint64(3)],
+                dtype="uint64",
+            ),
+        }
+    )
+
+    got = df.groupby("gb")[["i64", "u64"]].sum()
+
+    exp = pd.DataFrame(
+        {
+            "i64": pd.Series(
+                {
+                    "A": pd.Series([2**60, 5], dtype="int64").sum(),
+                    "B": pd.Series([7], dtype="int64").sum(),
+                },
+                dtype="int64",
+            ),
+            "u64": pd.Series(
+                {
+                    "A": pd.Series(
+                        [np.uint64(10), np.uint64(2**54)], dtype="uint64"
+                    ).sum(),
+                    "B": pd.Series([np.uint64(3)], dtype="uint64").sum(),
+                },
+                dtype="uint64",
+            ),
+        }
+    )
+    exp.index.name = "gb"  # align index name with groupby result
+
+    pd.testing.assert_frame_equal(got, exp)
+    assert got["i64"].dtype == "int64"
+    assert got["u64"].dtype == "uint64"
+
+
+def test_groupby_sum_dataframe_nullable_integers_min_count_by_column():
+    # Nullable Int64 / UInt64 with missing values; verify per-column min_count behavior
+    df = pd.DataFrame(
+        {
+            "gb": ["A", "A", "A", "B"],
+            "I": pd.Series([pd.NA, 2**60, pd.NA, 5], dtype="Int64"),
+            "U": pd.Series([pd.NA, np.uint64(7), pd.NA, np.uint64(2)], dtype="UInt64"),
+        }
+    )
+
+    out_na = df.groupby("gb")[["I", "U"]].sum(min_count=2)
+    assert out_na.loc["A", "I"] is pd.NA
+    assert out_na.loc["A", "U"] is pd.NA
+    assert out_na.loc["B", "I"] is pd.NA
+    assert out_na.loc["B", "U"] is pd.NA
+    assert out_na["I"].dtype.name == "Int64"
+    assert out_na["U"].dtype.name == "UInt64"
+
+    out_ok = df.groupby("gb")[["I", "U"]].sum(min_count=1)
+    assert out_ok["I"].dtype.name == "Int64"
+    assert out_ok["U"].dtype.name == "UInt64"
+    assert out_ok.loc["A", "I"] == 2**60
+    assert out_ok.loc["A", "U"] == np.uint64(7)
+    assert out_ok.loc["B", "I"] == 5
+    assert out_ok.loc["B", "U"] == np.uint64(2)