Between 1980 and 1993, collagenous colitis was diagnosed in 56 patients at UCLA. These authors evaluated the histology of almost 300 biopsies from these patients to further clarify the relationship between diagnostic histology and site of biopsy. As expected, their patients were predominately women (71%), with a mean age at diagnosis of 59 yr. Chronicity of diarrheal symptoms ranged from 3 months to 15 yr. Diagnostic subepithelial collagen thickness was variable throughout the colon. The highest yield was in transverse colon (83%), right colon (70%), and lowest in rectosigmoid (66%). Diagnostic collagen thickness was >15 microm in most biopsies, but was detected most readily when >30 microm--a threshold the authors suggest increases the diagnostic com-fort level. They recommend flexible sigmoidoscopy as adequate for diagnosis of most patients with collagenous colitis, noting colonoscopy is often used to clarify diagnosis, i.e., when only mild or focal colitis is seen distally.