The first German national study on the prevalence of nosocomial and community-acquired infections was performed in 1994 in medical, surgical, gynaecological/obstetrical and intensive care departments. 14,966 patients in 72 German hospitals representatively selected according to size were investigated by outside physicians. These were trained in the use of CDC definitions for nosocomial infections, and their diagnoses validated. Community-acquired infections were recorded according to the assessment of the hospital physicians. For the diagnosis of nosocomial infections, only the opinion of the outside investigators was decisive. A prevalence of 3.5% was found for nosocomial infections and 10.0% for community-acquired infections. The use of antibiotics was documented in 17.7% of all patients on the prevalence day. Of the patients undergoing antibiotic therapy, 16.9% had a nosocomial infection, 47.9% a community-acquired one. In the remaining 35.1% neither a nosocomial nor a community-acquired infection was confirmed.