Campylobacter fetus subspecies jejuni was isolated fom the feces of 63 (3.2%) of the 1,953 patients who had stools cultured at the Mayo Clinic in 1979. In contrast, Salmonella and Shigella combined were isolated from 31 (1.6%) patients. Two patients had double infections with Salmonella species and C. fetus subsp jejuni. Three patients had no diarrhea at the time of stool culture. One patient, who had chronic lymphocytic leukemia, had both blood and stool cultures positive for C. fetus subsp jejuni. There was a seasonal incidence that peaked in July when 7.8% of all patients who had stools cultured had C. fetus subsp jejuni isolated. Thirteen cases occurred in children 5 years of age and younger and 29 cases occurred between the ages of 15 and 30 years. Clinical features often included a prodrome of malaise, which preceded the onset of abdominal cramps, diarrhea, anorexia, fever, nausea, and vomiting. Grossly bloody diarrhea occurred in 33 patients, and massive intestinal bleeding occurred in 1 patient as a late complication after diarrhea had resolved. Transient splenomegaly was attributed to C. fetus subsp jejuni on one occasion. Proctoscopic findings may be similar to those seen in inflammatory bowel disease or pseudomembranous colitis. Three patients were referred to this institution with newly diagnosed chronic ulcerative colitis, and one patient was referred with newly diagnosed Crohn's disease. C. fetus subsp jejuni was isolated from their stools, and the diagnosis of inflammatory bowel disease was subsequently dropped. A selected review of cases illustrates the variety of gastrointestinal manifestations seen with this organism.