Studies to determine the diagnostic value of ileoscopy and biopsy are not available. In an attempt to clarify the role of this technique in the diagnosis of intestinal Crohn's disease, 110 patients with a radiological diagnosis of inflammatory disease of the terminal ileum were examined in a prospective study. Suspicion of Crohn's disease was rejected in 28 patients. In 18 patients the terminal ileum was normal, while 10 patients had lymphoid nodular hyperplasia. Endoscopic lesions with a predictive value of 0.96 were found in 25 of 48 patients with the final diagnosis of Crohn's disease. Diagnostic granulomas were only found in 4 patients, but lesions consistent with Crohn's disease were present in the pathology sections of 17 patients. It was concluded that ileoscopy with biopsy is a valuable tool in the diagnosis of inflammatory ileal disease and can provide useful information about the nature and extent of the inflammation.