Between October 1997 and June 2001, 140 English hospitals participating in the surveillance of surgical site infection (SSI) with the Nosocomial Infection National Surveillance Service (NINSS) reported 2832 SSIs following 67 410 surgical procedures in nine defined categories of surgery. Limb amputation had the highest incidence of SSI with 14.3 SSIs per 100 operations. For all categories of surgery, except knee prosthesis (P=0.128), there was a linear increase in the incidence of SSI when the American National Nosocomial Infections Surveillance risk index increased. Superficial incisional SSI was more common than deep incisional and organ/space SSI, and accounted for more than half of all SSIs for all categories of surgery. The postoperative length of stay (LOS) was longer for patients with SSI, and when adjusted for other factors influencing LOS, the extra LOS due to SSI ranged from 3.3 days for abdominal hysterectomy to 21.0 days for limb amputation, and was at least nine days for the other categories. The additional cost attributable to SSI ranged from pound959 for abdominal hysterectomy to pound6103 for limb amputation. Deep incisional and organ/space SSI combined incurred a greater extra LOS and cost than superficial incisional SSI for all categories of surgery, except limb amputation. The crude mortality rate was higher for patients with SSI for all categories of surgery but, after controlling for confounding, only patients with SSI following hip prosthesis had a mortality rate that was significantly higher than those without SSI [odds ratio (OR)=1.8, P=0.002]. However, the adjusted mortality rate for patients with deep incisional and organ/space SSI compared with those without SSI was significantly higher for vascular surgery (OR=6.8, P<0.001), hip prosthesis (OR=2.5, P=0.005) and large bowel surgery (OR=1.8, P=0.04). This study shows that the adverse impact of SSI differs greatly for different categories of surgery, and highlights the importance of measuring the impact for defined categories rather than for all SSIs and all surgical procedures.