College women with acute urinary tract infections were assigned randomly to three groups: 110 received trimethoprim (TMP), 300 mg once a day for seven days (group 1); 104 received TMP, 200 mg once a day for 10 days (group 2); and 108 received nitrofurantoin, 100 mg four times a day for 10 days (group 3). All primary urinary pathogens were sensitive to TMP, and 98.8% were sensitive to nitrofurantoin. A satisfactory clinical response was achieved by the end of therapy in each group. Bacteriologic cures occurred in 100.0% of the TMP groups and 99.0% of the nitrofurantoin group by the end of therapy. By four weeks after therapy, bacteriologic cure rates were 87.0% in group 1, 93.0% in group 2, and 82.0% in group 3. Bacteria responsible for the recurrences remained sensitive to TMP and nitrofurantoin. Patients with positive and negative tests for antibody-coated bacteria had similar responses to therapy. After therapy there was minimal emergence of Enterobacteriaceae resistant to TMP or nitrofurantoin in the fecal and urogenital flora. Both TMP and nitrofurantoin were tolerated well and were effective for treatment of acute urinary tract infections in women.