The clinical, radiological, and endoscopic aspects of Yersinia enterocolitica infections in man were studied in a group of 37 adult patients observed in a 4-year period in a single gastrointestinal unit. The diagnosis was based on isolation of the bacterium in all but 1 patient. Abdominal pain and diarrhea were the most prominent symptoms, occurring in 80% of the patients. A syndrome simulating appendicitis was observed in 40%. The duration of symptoms before diagnosis varied from 1 or 2 weeks in 32 patients to several months in 5. On radiological examination the terminal ileum was involved over a distance of 10 to 20 cm in 21 of 24 patients. A coarse, irregular, or nodular mucosal pattern and pictures suggestive of ulcerations were the most prominent and early radiological signs. Endoscopic observations in 13 patients with marked diarrhea showed signs of colitis in 6 and aphthoid ulcers in 2 patients. On pathological examination, ulcerations and a nonspecific acute inflammatory cell infiltrate were observed. Although treatment with tetracycline or chloramphenicol resulted in 4 to 6 weeks in the disappearance of most symptoms and signs, pictures of "follicular ileitis" persisted for several months.