To evaluate the relationship between duration of preoperative stay and the risk of nosocomial infection, we studied 449 patients who underwent surgery at the University Hospital of Granada during the first six months of 1986. Patients were chosen from two cross-sectional surveys. Nosocomial infection was studied throughout each patient's hospital stay. Other variables included preoperative stay, age, severity and total length of stay. The data were analyzed by comparing nosocomial infection for different lengths of preoperative hospitalization, age and severity by calculating the Odds Ratio. The effects of age and severity were studied by stratifying patients by duration of preoperative stay. Two multivariate regression models were used to confirm the results of the stratified analysis. The results suggest that lengthening the preoperative stay may increase the risk of nosocomial infection in surgical wounds and in other sites, and may simultaneously potentiate the effects of other risk factors such as age and severity of the patient's condition, whose influence on susceptibility to infection increases with the duration of preoperative stay.