During a 23-month study, we identified 1,233 patients with nosocomial urinary tract infections (UTIs) among 40,718 consecutive admissions by using a standardized, prospective system of hospital-wide surveillance. Nosocomial bloodstream infections (BSIs) occurred in 565 patients, 32 of whom had BSIs originating from UTIs, for an attack rate of 2.7 per 100 patients with nosocomial bacteriuria. Patients with UTIs due to Serratia marcescens were most likely to develop secondary BSIs (rate, 16 per 100) compared to patients with nosocomial UTIs due to other organisms (rate, less than or equal to 4.3 per 100; P less than 0.05). Furthermore, the median interval between documentation of UTI and of secondary BSI was 24 days for patients with infections due to S marcescens compared to one day for the entire group (P less than 0.005). Risk factor analysis indicated that men with UTIs were more likely to develop secondary BSIs than were women (P less than 0.05). Intensified infection control efforts are particularly necessary with high-risk groups such as bacteriuric men--especially among those patients with UTIs due to S marcescens--to reduce the incidence of secondary, hospital-acquired BSIs.