A five year prospective study of surgical wound infection complicating eight clean elective operations was carried out in 9,108 community hospital patients by detailed stratification of risk. Remote infection, diabetes mellitus and/or operations lasting beyond 4 hours characterized high risk patients with disparate surgical wound infection rates of 1.7 percent to 7.9 percent for individual operations. Absence of these three factors defined a low risk population with statistically similar rates of 0.8 percent to 2.8 percent for the different operations, with an over-all rate of 1.5 percent. Low and high risk definitions derived from observations in eight hospitals in 1975–1977 were predictive in 12 hospitals in 1978–1979. Both classes of patients with surgical wound infection had prolonged postoperative hospitalization. Staphylococcus aureus was recovered from 50 percent of the surgical wound infections in low risk patients with hernia repair, hip fracture repair, hip prosthesis, laminectomy and mastectomy operations and from 5 percent with cesarean section, femoropopliteal bypass and hip replacement procedures (P < 0.001). In nine high risk patients, bacteria recovered from remote infections were also present in surgical wound infections. Comparison of the occurrence of surgical wound infections in clean operations in different hospitals may be made more meaningful by stratification of risk factors and analysis of expected infecting bacteria.