Four therapeutic regimens of sulfisoxazole were compared and contrasted with the antibody-coated bacteria test in patients with acute urinary tract infections. Of 158 college coeds who entered the study 146 completed the randomly assigned regimen. All 146 patients received 2 gm. sulfisoxazole initially and 1 gm. 4 times daily for 3 days in 44 patients (group 1), 7 days in 51 (group 2), 14 days in 29 (group 3) and 21 days in 22 (group 4). The presumptive sites of infection by the antibody-coated bacteria test were kidney (positive test) in 43 per cent of the patients and bladder (negative test) in 51.3 per cent. There was no correlation between the results of the antibody-coated bacteria test with either the presenting symptoms or the therapeutic responses. The bacteriologic cure rates at 2 days after therapy were 100 per cent in all groups and at 4 weeks after therapy they were 88.6 per cent in group 1, 86.3 per cent in group 2, 86.2 per cent in group 3 and 91 per cent in group 4. A 3-day course of sulfisoxazole was as effective as the longer regimens.