An endoscopical and histological study was carried out in 114 patients, treated by 'curative' resection of the terminal ileum and part of the colon for Crohn's disease, in order to study the natural history of recurrent Crohn's disease. The recurrence rate of Crohn's disease in patients examined within one year of the operation was 72%. This figure did not differ significantly from that in patients examined one to three years or three to 10 years after surgery (79% and 77% respectively). Recurrence was located in the neoterminal ileum and at the anastomosis in 88% of the patients. Early endoscopic signs of recurrence were small aphthous ulcers in the neoterminal ileum. Ileal biopsies at this stage showed an important inflammatory cell infiltrate of the lamina propria with numerous eosinophils and fusion and blunting of the villi. More advanced lesions observed in patients examined one to three years after surgery, consisted of larger, often serpiginous ulcerations and nodular thickening of folds. In patients examined three to 10 years after the operation, the anastomosis was frequently stenosed and rigid, with large ulcers extending from the stenosis into the colon. Mucosal granulomas may be found in normal appearing mucosa as well as in the obviously inflamed mucosa surrounding the ulcers. These studies suggest that recurrence of Crohn's disease almost always develops in the first year after the operation. Significant endoscopic lesions may be present without clinical symptoms, particularly in the earlier stages of the disease.