We investigated the diagnostic value of biopsies taken from Crohn's lesions such as ulcers, aphthoid lesions, cobble-stone epithelium and "pseudopolyps". One hundred and forty-six colonoscopies performed in 141 patients with Crohn's disease (CD) were analyzed. Biopsies were taken during colonoscopy from different gross lesions. Histologic confirmation of CD by granulomas and microgranulomas was obtained in 36 cases from 146 colonoscopies (24.7%). In 80 investigations (54.8%) the histologic findings were consistent with, but not diagnostic of, CD, in 30 cases (20.5%) histology was non-diagnostic. The lesions most likely to contain granulomas were ulcers and we therefore conclude that biopsies taken from ulcer are diagnostically superior to those taken from other lesions seen in CD.