In Crohn's disease (CD), aphthous lesions are regarded as possible precursors of typical intestinal involvement. To determine the natural course of intestinal lesions in CD of aphthous type, the clinical course of 10 patients was retrospectively investigated during a period ranging from 6 to 16 years after diagnosis. The criterion for inclusion was confirmed aphthous lesions within the gastrointestinal tract with histologically verified epithelioid granuloma. The degrees of aphthous lesions in the small intestine and the colon were graded by small bowel radiography, barium enema examination and colonoscopy. Five patients developed typical CD during a period ranging from 0.8-3.3 years. The site of involvement was the ileum in three patients, the colon in one patient and both the ileum and the colon in one patient. Typical small intestinal CD occurred in four of seven patients with marked aphthous lesions of the small intestine, whereas colonic CD occurred in two of eight patients with such aphthous lesions of the colon. These findings suggest that CD of aphthous type is not necessarily a precursor of clinically overt disease. This may especially be the case for colonic aphthous lesions.