A "catheter team", consisting of two hospital assistants specially trained to catheterize male patients, inserted indwelling catheters in 435 men over a two-year period. The infection rate was 33%; in the 200 patients not treated with antimicrobial drugs (study group) the rate was 37%, while in the 235 patients who were so treated (antibacterial group) the infection rate was 29%. Fifty percent of patients not treated were infected after 6.3 days, whereas in patients on antibacterial therapy a 50% infection rate was not reached until 14 days after insertion. Therefore, no antibacterial therapy is necessary if it is anticipated that the catheter will be necessary for less than four days. On the other hand, prophylactic antibacterial therapy would delay the onset of infection considerably if catheterization were expected to continue for more than four days. Sulfisoxazole was our drug of choice for prophylactic treatment.