The aims of this study were to determine adherence to the perioperative antibiotic prophylaxis (PAP) protocol used at a large Italian teaching hospital during a 6-year period, to assess the variables associated with inappropriate administration, and to measure the impact on surgical site infection (SSI) rates. There were 28 621 patients surveyed of which 74·6% received PAP. An improvement in adherence to the PAP protocol was registered for 58·8% of patients. Significant risk factors were an American Society of Anesthesiologists (ASA) score ≥ 2 [odds ratios (OR) from 1·28 (95% confidence interval (CI) 1·19-1·37) to 1·87 (95% CI 1·43-2·44)], prolonged duration of surgery (OR 1·68, 95% CI 1·56-1·82) and urgent surgery (OR 2·16, 95% CI 1·96-2·37). During the study period, a significant reduction in SSIs rates was detected. We concluded that the global reduction of inadequate PAP administration signifies the efficacy of a multidisciplinary quality improvement initiative on antimicrobial utilization, and this is supported by the observed reduction of the SSI rate.