A study was made of quantitative urine cultures in bacteremic patients with urinary tract infection as the only identifiable source. These patients represent the most serious form of urinary tract infection and should be recognized as significant by the criteria used for interpreting quantitative urine cultures. In 83 patients, 15 (18%) had quantitative urine counts less than 10(5) colony-forming units (CFU)/mL. Ten (12%) had counts between 10(4) and 10(5) CFU/mL. The incidence of patients with less than 10(4) CFU/mL (5%) is probably a low estimate because of the urine culture methods used, and this requires further study. The results of this study support the view that significant urinary infections may be associated with quantitative urine counts of less than 10(5) CFU/mL.