To investigate the value of colonoscopy in the diagnosis of acute infectious colitis, we prospectively cultured both feces and biopsy specimens obtained during colonoscopy of patients who, because of clinical features such as the acute onset of abdominal pain and diarrhea, were suspected of having the disease. Of the 20 patients who participated in the study, some causative micro-organism was identified in 13 (Campylobacter in 6 patients, Salmonella in 5 patients, and Yersinia in 2 patients), but not in the remaining 7. In addition, biopsy specimens were more sensitive for culture (positive for Campylobacter in 4 patients, Salmonella in 5 patients, and Yersinia in 1 patient) than were feces samples (positive for Campylobacter in 2 patients and for Yersinia and Salmonella in 1 patient each) (50% versus 20%, p = 0.048). These findings suggest that cultures of biopsy specimens obtained during colonoscopy may be diagnostic in sporadic cases of acute enterocolitis caused by bacteria.