The aim of this work is to prospectively evaluate the incidence of infection, from June 1986 to June 1987, in 640 patients submitted to surgical treatment at the São Francisco Hospital, in Ribeirão Preto, São Paulo, Brazil. The overall incidence of infection was 10.31%. The incidence of wound abscess was 6.25%, and urinary infection 5.75%. In the surgical procedures considered as clean, the infection rate was 8.62%, in the clean-contaminated 14.81%, in the contaminated 8.33%, and in the dirty 16.94%. The antimicrobian drugs contributed to increase the infection rate. The hospital infection rate for the patients at infirmaries was 10.88%, and for the patients at private rooms 4.92%. The mortality rate due to hospital infection was 12.12%. The authors stress that a constant attention with the hospital infection is needed to verify the infection rate to be able to make a control program of the asepsis, antisepsis and sterilization methods, as well as to improve the operative techniques and the patient's management during the pre, per and postoperative period.