During the last week of May 1986, a 1-week prospective study on antibiotic utilization in surgical patients was held in 104 (42%) of the 247 Belgian acute care hospitals. All surgical patients with a post-operative stay of at least 3 days were studied, involving 3112 patients. Each patient was observed for 7 days, starting from the day before surgery. Antibiotics were administered to 71·9% of all patients; 21·9% received therapeutic antibiotics and 52·9% prophylactic antibiotics; 2·9% received both. Of the 1285 patients undergoing a surgical procedure with no indication for antimicrobial prophylaxis, 50·7% nevertheless received prophylaxis; 92·8% of patients with a generally recognized indication for prophylaxis received antibiotic prophylaxis. Less than one fifth (17·1 %) of all prophylactic courses were stopped on the day of the intervention whilst 26·3% were continued up to the fifth post-operative day or beyond. The most frequently prescribed drugs for this indication included first and second generation cephalosporins and nitroimidazoles. The number of different generic drugs utilized per hospital ranged from 1 to 18 (mean: 7·7).