Two regimes of rifaprim (RPM), a 3.75 to 1 combination of rifampicin (RIF) and trimethoprim (TMP), were compared with co-trimoxazole (CO-T) for the treatment of urinary tract infection in 60 patients. Dosages were: A, 450 mg RIF + 120 mg TMP twice daily; B, 600 mg RIF + 160 mg TMP at bedtime and C, 800 mg sulphamethoxazole (SMZ) + 160 mg TMP twice daily. Clinical results were similar but CO-T treatment was accompanied by a greater number of late bacteriological failures. In none of 40 patients receiving RPM did resistance to any of the components develop, while in three of 20 patients receiving CO-T resistance to TMP or SMZ emerged during treatment. There were no side effects in the patients receiving RPM, but three patients developed transient laboratory abnormalities. One patient receiving CO-T had a rash and one further transient laboratory abnormalities. Satisfactory concentrations of RIF and TMP were measured in urine up to 24 h after a dose of 600 mg RIF + 160 mg TMP. RIF may be a better companion to TMP than the sulphonamides for the treatment of urinary tract infection.