Trimethoprim, in combination with sulfadiazine or sulfamethoxazole was administered orally for 7 to 14 days to 84 dogs with urinary tract infections (UTI). The daily dosage of 26.4 mg/kg (12 mg/lb) was divided into 2 equal parts and administered at about 12-hour intervals. Response to treatment, based on negative urine culture during or after therapy, was 37 of 45 (82%) for UTI caused by Escherichia coli, 11 of 15 (73%) UTI caused by Proteus mirabilis, 8 of 12 (67%) UTI caused by Klebsiella pneumoniae, 6 of 6 (100%) UTI caused by Staphylococcus aureus, and 5 of 9 (56%) UTI caused by Streptococcus spp. These 5 species comprised 88% of the bacteria isolated from the dogs in this study.