This study evaluates the importance of low-colony-count bacteriuria (less than 10(5) CFU/mL) in septicemia originating from urinary tract infections. In a 14-month period, 260 episodes of septicemia occurred. No clinical or microbiologic evidence for a source other than the urinary tract was evident in 68 (26.2%) cases. Of these 68 patients, 13 (19.1%) had colony counts less than 10(5) CFU/mL, and 6 of the 13 had colony counts less than 10(4) CFU/mL. Nine of the infections were community acquired and four were nosocomial. None of the nosocomial cases were associated with an indwelling catheter; four of the 13 patients were receiving chemotherapy and/or steroid therapy. These data support the thesis that some cases of septicemia in patients other than acutely dysuric women, can be caused by UTIs with low colony counts.