The efficacy of the traditionally recommended ampicillin (Amp) plus gentamicin (GM) regimen was compared with that of a trimethoprim-sulfamethoxazole (TMP/SMZ)-plus-GM regimen and the adequacy of 14 days total therapy for acute uncomplicated pyelonephritis (AUPN). Eighty-five women hospitalized for AUPN were randomly assigned to receive either Amp, 1 g intravenously (iv) every 6 h for 3 days, then 500 mg orally four times daily, or TMP/SMZ, 160/800 mg iv every 12 h for 3 days, then 160/800 mg orally twice daily. Initially, all patients also received GM every 8 h iv (mean, 606 doses). Antimicrobial resistance necessitated modifying therapy of 14 (32%) of the Amp recipients but of none of the TMP/SMZ recipients (P less than .001). Both regimens produced a satisfactory bacteriologic and clinical response in all cases. Reinfection occurred in 11% of Amp and in 8% of TMP/SMZ recipients. No patient experienced relapsing infection. The TMP/SMZ regimen was less costly and less likely to require modification due to antimicrobial resistance.