To describe the epidemiology of bacteremia in a large, well defined population, the authors reviewed medical records for residents of Charleston County, South Carolina, who had bacteria isolated from blood in the period 1974 to 1976. The incidence was 80 cases per 100,000 population per year. The most common organisms were Escherichia coli, Staphylococcus aureus, Klebsiella, and Streptococcus pneumoniae. The incidence was highest for neonates, infants, and those 70 years of age and older with annualized attack rates of 1,864,250, and 446 cases per 100,000 population, respectively. The incidence was 3.2 times higher for blacks than for whites and, within races, appeared to be independent of family income. Twenty-five per cent of patients had no clinically apparent focus of infection, 26% had urinary tract infection, and 17% had pneumonia. Thirty-nine per cent of cases were nosocomial, and 30% of patients died.