Clostridium difficile-associated diarrhea (CDAD) is regarded as an emerging nosocomial infection. All patients positive for C. difficile in Sweden were recorded during 1995, including primary care patients. Those positive for toxin in feces were defined as CDAD cases. A total of 5,133 CDAD cases were recorded (58 per 100,000 inhabitants per year), as compared with 86 cases diagnosed in 1978 and 553 in 1983. CDAD was almost twice as prevalent as all (combined) diagnosed domestic cases of reportable bacterial and protozoal diarrhea. The age-specific incidence was little affected by gender but increased > 10-fold over the age range of 60-98 years. The differences in overall CDAD incidence were sixfold between counties and threefold between major hospitals. Among hospitalized patients the incidences were highest in geriatric/rehabilitation wards, followed by infectious diseases and internal medicine wards; 28% of all cases involved no recent hospitalization and were defined as community-acquired CDAD.