diff --git a/tests/cpp/tree/hist/test_histogram.cc b/tests/cpp/tree/hist/test_histogram.cc
index ff721cf129bd..8acf0959fe70 100644
--- a/tests/cpp/tree/hist/test_histogram.cc
+++ b/tests/cpp/tree/hist/test_histogram.cc
@@ -2,7 +2,11 @@
  * Copyright 2018-2021 by Contributors
  */
 #include <gtest/gtest.h>
+
 #include "../../helpers.h"
+#include "../../categorical_helpers.h"
+
+#include "../../../../src/common/categorical.h"
 #include "../../../../src/tree/hist/histogram.h"
 #include "../../../../src/tree/updater_quantile_hist.h"
 
@@ -311,9 +315,72 @@ TEST(CPUHistogram, BuildHist) {
   TestBuildHistogram<double>(false);
 }
 
+namespace {
+void TestHistogramCategorical(size_t n_categories) {
+  size_t constexpr kRows = 340;
+  int32_t constexpr kBins = 256;
+  auto x = GenerateRandomCategoricalSingleColumn(kRows, n_categories);
+  auto cat_m = GetDMatrixFromData(x, kRows, 1);
+  cat_m->Info().feature_types.HostVector().push_back(FeatureType::kCategorical);
+  BatchParam batch_param{0, static_cast<int32_t>(kBins)};
+
+  RegTree tree;
+  CPUExpandEntry node(RegTree::kRoot, tree.GetDepth(0), 0.0f);
+  std::vector<CPUExpandEntry> nodes_for_explicit_hist_build;
+  nodes_for_explicit_hist_build.push_back(node);
+
+  auto gpair = GenerateRandomGradients(kRows, 0, 2);
+
+  RowSetCollection row_set_collection;
+  row_set_collection.Clear();
+  std::vector<size_t> &row_indices = *row_set_collection.Data();
+  row_indices.resize(kRows);
+  std::iota(row_indices.begin(), row_indices.end(), 0);
+  row_set_collection.Init();
+
+  /**
+   * Generate hist with cat data.
+   */
+  HistogramBuilder<double, CPUExpandEntry> cat_hist;
+  for (auto const &gidx : cat_m->GetBatches<GHistIndexMatrix>(
+           {GenericParameter::kCpuId, kBins})) {
+    auto total_bins = gidx.cut.TotalBins();
+    cat_hist.Reset(total_bins, {GenericParameter::kCpuId, kBins},
+                    omp_get_max_threads(), 1, false);
+    cat_hist.BuildHist(0, gidx, &tree, row_set_collection,
+                        nodes_for_explicit_hist_build, {}, gpair.HostVector());
+  }
+
+  /**
+   * Generate hist with one hot encoded data.
+   */
+  auto x_encoded = OneHotEncodeFeature(x, n_categories);
+  auto encode_m = GetDMatrixFromData(x_encoded, kRows, n_categories);
+  HistogramBuilder<double, CPUExpandEntry> onehot_hist;
+  for (auto const &gidx : encode_m->GetBatches<GHistIndexMatrix>(
+           {GenericParameter::kCpuId, kBins})) {
+    auto total_bins = gidx.cut.TotalBins();
+    onehot_hist.Reset(total_bins, {GenericParameter::kCpuId, kBins},
+                      omp_get_max_threads(), 1, false);
+    onehot_hist.BuildHist(0, gidx, &tree, row_set_collection,
+                          nodes_for_explicit_hist_build, {},
+                          gpair.HostVector());
+  }
+
+  auto cat = cat_hist.Histogram()[0];
+  auto onehot = onehot_hist.Histogram()[0];
+  ValidateCategoricalHistogram(n_categories, onehot, cat);
+}
+}  // anonymous namespace
+
+TEST(CPUHistogram, Categorical) {
+  for (size_t n_categories = 2; n_categories < 8; ++n_categories) {
+    TestHistogramCategorical(n_categories);
+  }
+}
+
 TEST(CPUHistogram, ExternalMemory) {
   size_t constexpr kEntries = 1 << 16;
-
   int32_t constexpr kBins = 32;
   auto m = CreateSparsePageDMatrix(kEntries, "cache");
   std::vector<size_t> partition_size(1, 0);
@@ -358,8 +425,8 @@ TEST(CPUHistogram, ExternalMemory) {
     size_t page_idx{0};
     for (auto const &page : m->GetBatches<GHistIndexMatrix>(
              {GenericParameter::kCpuId, kBins, hess})) {
-      multi_build.BuildHist(page_idx, space, page, &tree,
-                                  rows_set.at(page_idx), nodes, {}, h_gpair);
+      multi_build.BuildHist(page_idx, space, page, &tree, rows_set.at(page_idx), nodes, {},
+                            h_gpair);
       ++page_idx;
     }
     ASSERT_EQ(page_idx, 2);