Carumonam and gentamicin were compared in a prospective, randomized study of 52 patients with complicated urinary tract infections. Patients were treated with either carumonam (1 g every 8 h) or gentamicin (1 mg/kg every 8 h). The mean duration of therapy (carumonam, 8.5 days; gentamicin, 8.5 days) was similar for both groups. A total of 45% of patients treated with carumonam and 48% of those receiving gentamicin were cured, as defined by a negative culture 1 to 2 weeks after therapy. After 4 to 6 weeks, the figures were 27% for carumonam and 38% for gentamicin. In the carumonam group, there were 6 relapses and 11 reinfections. In the gentamicin group, there were eight relapses and five reinfections. Adverse effects in the carumonam group were limited to phlebitis at the intravenous infusion site in two patients; another patient developed bloody diarrhea. Nephrotoxicity was documented in two patients in the gentamicin treatment group (9%), and another patient developed minor liver function disturbances. Three patients with gentamicin-resistant carumonam-susceptible isolates were treated with carumonam, and two were cured. Urinary colonization with group D streptococci occurred in 7 of 27 (26%) carumonam-treated patients compared with 7 of 19 (37%) gentamicin-treated patients; no one required treatment. A significant correlation was found between colonization with group D streptococci and neurogenic bladder dysfunction (P less than 0.007). It is concluded that the use of the carumonam is as effective as the gentamicin regimen in the treatment of complicated urinary tract infections.