433 episodes of E. coli bactereaemia over a 5-year period in a Danish university hospital were studied with special emphasis on possible differences between nosocomial (NO) and community-acquired (CA) cases. Data from 186 males and 247 females with ages ranging from 9 days to 94 years were recorded. The average incidence of E. coli bacteraemia was 24.4 episodes/10,000 admissions/year. Older females accounted for the largest number of cases, which reflected the composition of the background population. The highest risk of infection was in males 80-89 years of age. The highest frequency of NO infections was in the departments of Intensive Care (90%) Orthopaedic Surgery (87%) and Haematology (80%). The most common focus was the urinary tract, with 72% of the episodes with a known focus, while the biliary tract was the focus in 14%. NO bacteraemia was independently related to immunosuppressive therapy, presence of predisposing factors, polymicrobial bacteraemia and presence of a non-urinary tract focus. A urinary tract focus was associated with CA bacteraemia, monomicrobial infection, female sex and a normal or elevated total white blood cell count. Patients with NO bacteraemia had predisposing factors more often than had CA patients, especially haematological malignancies and immunosuppressive therapy. Lack of a known bacterial focus was more common in NO than CA episodes, particularly among patients with haematologic malignancies. The overall mortality was 21%. Increased mortality was independently related to leukopenia (45%), immunosuppressive therapy and NO bacteraemia.