A prospective study was carried out in a medical and surgical ICU to determine the incidence of nosocomial urinary tract infection (NUTI) and to identify the most important risk factors. Over a 6 month period, 180 patients were included. All had an indwelling catheter. Six risk factors were studied: age, sex, illness (medical, surgical, trauma), hospital or extra-hospital origin, simplified acute physiology score and length of bladder catheterization. Forty three patients developed a NUTI. Length of bladder catheterization was the only significant different risk factor in infected and non-infected patients. Kaplan Meir analysis was used to determine time to development of NUTI. The risk rose from 19% for 5 day long catheterization to 50% for 14 day long catheterization.