This prospective study was carried out to investigate the risk factors and incidence of surgical site infection (SSI) among patients in surgical wards of five hospitals affiliated to Iran University of Medical Sciences. Data was collected in a register card filled by specially trained staff. Nine-hundred and eighteen patients admitted in surgical wards were followed 30 days postoperatively for SSI during 1 April 2003 to 30 September 2003. A total of 77 patients were identified among 918 cases included in the study, with a resulting overall SSI rate of 8.4%. The risk of SSI was increased by age older than 60 years (OR = 3.9; P < 0.0001), diabetes mellitus (OR = 4.9; P < 0.0001), smoking (OR = 3.1; P < 0.0001), obesity (OR = 4.1; P < 0.0001) and wound drain (OR = 2.2; P < 0.0001). There were significant statistical difference in duration of anaesthesia (131.6 vs. 177 min, P < 0.001) and duration of surgery (99 vs. 140.5) between patients without SSI and patients with SSI. In conclusion, identification of the risk factors for SSI will help physicians to improve patient care and may decrease mortality and morbidity and hospital care costs of surgery patients.