Lymphocytic colitis (LC) is classically described as a triad of chronic nonbloody, watery diarrhea, normal or nearly normal endoscopy findings, and colonic epithelial lymphocytosis without a thickened subepithelial collagen table (SECT). It is unknown how often patients with colonic epithelial lymphocytosis without a thickened SECT actually present with this classic triad. Cases diagnosed histologically as lymphocytic or microscopic colitis were reviewed. Criteria for inclusion were the presence of at least 15 surface lymphocytes per 100 epithelial cells and the absence of a thickened SECT (<12 microm). Clinical features and course were recorded by chart review and telephone follow-up. Forty patients met the inclusion criteria, including 25 women and 15 men with a mean age of 63.2 years (range, 25-83 years). Twenty-eight patients had the classic triad and were designated as having classic LC. The other 12 patients fulfilled the histologic criteria but not the clinical or endoscopic criteria for classic LC and were classified as having atypical LC (constipation, five patients; macroscopic colitis at endoscopy, five patients; hematochezia, one patient; and incidental finding, one patient). Clinically, patients with classic LC were predominantly women and had a higher incidence of autoimmune disease (p = 0.03) than did those with atypical LC. Histologically, surface eosinophilia was significantly greater in patients with classic LC (p = 0.04). Twenty patients were using nonsteroidal antiinflammatory drugs at the time of their colonic biopsy. Surface epithelial lymphocyte counts were higher in these patients, particularly in the distal sigmoid colon (p = 0.02). Fourteen patients had associated autoimmune disease, including three patients with sprue diagnosed by small bowel biopsy, all of whom responded to gluten withdrawal. Diarrhea present in 25 patients, without documented evidence of celiac sprue, was self-limited in five, resolved with treatment in three, required intermittent treatment in eight, daily treatment in five, and was refractory to treatment in four. All eight patients who experienced spontaneous or treatment-related symptom resolution had classic LC. No histologic feature correlated with clinical course. In conclusion, our study shows that colonic epithelial lymphocytosis without a thickened SECT is a histologic finding seen in a heterogeneous group of patients. Within this heterogeneous group is a distinct subset of patients who have the classic clinicopathologic triad of LC. This subset of patients has striking similarities to patients with collagenous colitis, lending further support to a close relationship between these two entities. Atypical LC comprises a heterogeneous group and includes patients with idiopathic constipation, coexisting LC and inflammatory bowel disease, and possibly infectious colitides. Because of the clinical heterogeneity among our study population, the descriptive term colonic epithelial lymphocytosis may be a more prudent diagnosis than lymphocytic colitis in the absence of adequate clinical information.