Rectal biopsies from a series of 35 patients with radiologic evidence of Crohn's disease were assessed histologically and classified according to the type of lesions identified. These were then correlated with the sigmoidoscopic findings and site of radiologic involvement. Overall, 40% of the biopsies were abnormal, and in 28% the changes were highly suggestive of Crohn's disease. Granulomas were identified in 15%. Differences between sigmoidoscopic interpretation and histologic diagnosis were present in 11% of cases. This yield of positive biopsies is similar to earlier reports but is approximately double the numbers recorded by the National Cooperative Crohn's Disease Survey. It is concluded that rectal biopsy may confirm the diagnosis in unselected cases of Crohn's disease and provide useful information in patients where the diagnosis was not previously established. In our hands rectal biopsy was an extremely safe diagnostic procedure.