We reviewed colorectal biopsies and clinical records from 36 patients with chronic watery diarrhea who had been diagnosed as having microscopic colitis and compared their histologic features with the more detailed and precise criteria for lymphocytic colitis. Published pathologic criteria for lymphocytic colitis were applied to the biopsies and compared. Focal or diffuse nature of the lymphoid infiltrate were noted separately. The focal lymphoid infiltrate was related to lymphoid aggregates in the lamina propria of the mucosa. Eighteen cases had focal lymphoid cell infiltration, and 16 of them had associated diverticula, polyps, or both. Eighteen cases had diffuse lymphoid cell infiltration, and six of them had diverticula or polyps. Results indicate that focal cellular infiltration strongly predicts associated diverticula or polyps. The group with no diverticula or polyps most closely conformed to histologic criteria for lymphocytic colitis (Kruskal-Wallis P < 0.02). We conclude that lymphocytic colitis comprises a well-defined group of cases within the large and less-defined group of microscopic colitis.