We studied 110 catheterized patients to determine the concentration of microorganisms that would indicate infection in urine aspirated from an indwelling urethral catheter. High-level bacteriuria or candiduria (greater than 10(5) colony-forming units per milliliter) developed in 34 patients. However, low-level bacteriuria or candiduria (less than 10(5) organisms per milliliter), which developed in 41 patients, progressed to concentrations above 10(5) organisms per milliliter 96 per cent of the time (P less than 0.001), usually within three days of the initial culture showing growth, unless the patient received intercurrent suppressive antimicrobial therapy. We conclude that the urinary tract of catheterized patients is highly susceptible to infection once small numbers of microorganisms gain access and that a concentration considerably below 10(5) organisms per milliliter may be clinically and epidemiologically important in this setting.