Pregnant patients with symptomatic and asymptomatic urinary tract infections were treated with a long and a short antibiotic regimen. Two hundred two patients were randomized prospectively to a single oral dose of 3.5 g ampicillin plus 1 g probenecid (98 patients) versus 500 mg ampicillin orally four times a day for 10 days (104 patients). The multiple-dose cure rate was statistically significantly better than that of the single-dose regimen (67.3% versus 57.1%, respectively). Interestingly, for resistant organisms, the cure rate for the long and short regimens was similar (48% versus 43%, respectively). In vitro susceptibility testing does not appear to be a good predictor of cure, at least for the single-dose group. Single-dose therapy with ampicillin and probenecid does not provide an optimal cure rate or prevent reinfection during pregnancy. Possible reasons for these findings are discussed.