During a 15-month study, 8097 fecal specimens submitted to clinical microbiology laboratories at eight hospitals in different parts of the United States were examined. Campylobacter jejuni was isolated from 4.6%, Salmonella from 2.3%, and Shigella from 1.0%. Isolation rates for each pathogen were highest from stool specimens that were watery, bloody, or contained leukocytes. The peak isolation rate for C. jejuni was in persons ages 10 to 29 years; for Salmonella, in children younger than age 10 years; and for Shigella, in children ages 5 to 9 years. The clinical features of the three infections were nearly identical. In contrast, abdominal pain, bloody diarrhea, fever, tenesmus, and abnormal sigmoidoscopy findings were present significantly more often in patients infected with C. jejuni than in a control group of patients with diarrhea. Of patients with leukocytes in their stools and a history of fever, 45.9% were infected with one of the three pathogens. Use of laboratory and clinical findings defined groups with high or low risk of these three infections but could not accurately predict isolation. Fecal cultures had the highest yields when obtained from patients within 7 days from the onset of symptoms.