A program for monitoring nosocomial infections was implemented in the surgical units of five hospitals. The overall incidence of infections after clean surgery varied from 6.6% to 15% depending on the units. Surgical wound infections were in the 2.8 to 5% range, which corresponded to 30-66% of nosocomial infections. The main septic complication of surgery in 2 of the 5 units was urinary tract infection, with an incidence of 1.6 to 7.8%. Bacteriaemia complicating urinary tract infections and intravenous catheter infections accounted for 4% to 16% of all infections. Staphylococcus aureus was responsible for 40 to 77% of surgical wound infections and Escherichia coli for 30 to 50% of urinary tract infections. After a 4-5 months period of monitoring, the incidence of nosocomial infections was reduced by 32 to 68% depending on individual units.