Lymphocytic and collagenous colitis, uncommon entities that are identifiable by histopathologic examination only, and intestinal spirochetosis may cause chronic diarrhea. Data on incidence, histopathology, and diseases associated with these infrequent forms of colitis are reviewed. Etiology and pathogenesis of lymphocytic and collagenous colitis are still unknown, but immuno-inflammatory factors may be involved in both forms as well as, in collagenous colitis, primary disorders of collagen metabolism or drug-induced toxic effects. The significance of intestinal colonization by spirochetes is disputed; however, recent results indicate that these microorganisms are capable of inducing disease.