In a survey of infections and antibiotic usage at Boston City Hospital in January 1970, 20% of the patients had infections considered to be community-acquired and 12% had infections considered to be acquired in the hospital. These prevalence ratios did not differ greatly from those found at this hospital in 1964 and 1967. There was no significant change since 1967 in the prevalence of Staphylococcus aureus in nosocomial infections. The most striking change noted was in the prevalence of Klebsiella-Enterobacter and Pseudomonas aeruginosa, which together, were isolated from 57% of all nosocomial infections. Antimicrobial agents were being administered to one third of patients at the time of the survey; penicillin G and ampicillin were the ones most frequently used. Periodic prevalence surveys have been invaluable in defining problems and trends in nosocomial infections and antibiotic usage at Boston City Hospital.