In a double-blind investigation of urinary tract infections in general practice the effects of sulphamethizole, trimethoprim and sulphadiazine-trimethoprim (co-trimazine) were compared. The investigation was conducted as a multi-practice study with the participation of 314 general practitioners. A total of 965 patients were treated and followed up. The results, evaluated by urine culture 2 weeks after commencement of treatment, showed that sterile urine was obtained in 68.5% of the patients treated with sulphamethizole, in 78.9% of those treated with trimethoprim, and in 83.3% of those treated with co-trimazine. Skin reactions occurred with sulphamethizole in 1.4% with trimethoprim in 4.1%, and with sulphadiazine-trimethoprim in 3.2%. We believe that rapidly excreted sulphonamides should continue to be preferred as drugs of first choice in the treatment of urinary tract infections in general practice, but that the use of trimethoprim or the combination sulphadiazine-trimethoprim is a worthwhile alternative to sulphonamide treatment, particularly for elderly patients and patients infected with sulphonamide-resistant bacteria.