Antibiotic-associated colitis is known to occur with temporal and geographic clustering. During 1979 a marked increase in the incidence of this disease was noted at our institutions. The clinical, epidemiological, and endoscopic features are reported. Typical features included the acute onset of watery diarrhea, fever, abdominal pain, tenderness, and the presence of leukocytosis. Epidemiological data revealed a nosocomial pattern of this illness and a striking female predominance. Two-thirds of the patients had an underlying gynecological or urological disorder. Dysuria and sterile pyuria were unexpected findings. It is suggested that the organism responsible for this disease may be hospital acquired and associated with underlying genitourinary disorders.