This paper addresses the problem of hospital stay length as a risk factor for nosocomial infection and as a modifier of the effect of other risk factors for hospital infection. Patients were selected form two cross-sectional studies done in two different seasons of 1986. Risk of infection rose fairly steadily as hospital stay length increased (correlation coefficient: 0.83, p less than 0.01). Several risk factors (operation, underlying disease, and age) were analyzed on the basis of 1) raw data and 2) data stratified by length of stay. The results showed that hospital stay length is a strong modifier of the remaining risk factors, generally reducing, their effect on the development of hospital infection as length of stay increases.