Lymphocytic colitis, formerly called microscopic colitis, is a clinicopathologic syndrome with chronic watery diarrhea and diffuse mucosal inflammatory changes with prominent intraepithelial lymphocytes. The 18 lymphocytic colitis patients studied presented with chronic watery diarrhea at a mean age of 53.8 +/- 17 years (+/- 1 SD). Roentgenographic, endoscopic, and culture data were not diagnostic. In patients tested, there was a high prevalence of arthritis (82%) and autoantibodies (50%) but no increase in frequency of histocompatibility antigens associated with well-defined autoimmune disease (DR3, B8). Lymphocytic colitis patients were compared to 21 patients with collagenous colitis. Similarities included age, symptomatology, and nondiagnostic radiographic and endoscopic studies. However, the sex distribution was statistically different, with an equal male-to-female ratio in lymphocytic colitis and female predominance (80%) in collagenous colitis. Other differences included dissimilar histocompatibility phenotypes and collagen band on biopsies of collagenous but not lymphocytic colitis. These findings suggest that lymphocytic and collagenous colitis may be related yet distinct disorders.