A prospective microbiologic evaluation of the urine was conducted on 100 catheterized ICU patients. Eight subjects had positive initial bladder urine cultures and were eliminated from further study. The remaining 92 patients were catheterized for up to 22 days, mean 4.8 ± 4.3 (SD). During the first 5 days, 57 (62%) patients completed the surveillance protocol; seven (12.3%) of these had bladder bacteriuria. Of 20 patients with positive urine cultures, six had microorganisms in the collection bag urine alone. The remaining 14 had organisms both in the bladder and in the bag urine. The urine collection bag was the apparent source of microorganisms in the bladder urine in only three cases. The daily incidence of new cases and the cumulative rate of bladder bacteriuria remained below 7% and 22%, respectively, during the first week of catheterization. These rates are similar to those previously reported for various other patient groups outside the ICU setting. Frequent microbiologic monitoring of the urine seems to be unnecessary for most ICU patients because of the short duration of catheterization.