A prospective study was carried out in which quantitative clean-voided urine cultures, were obtained at the first clinic visit from 986 consecutive pregnant women, followed to delivery to assess the maternal and pediatric finding associated with maternal asymptomatic bacteriuria. Patients to be evaluated were divided in two groups: Treatment group (TG) with 42/46 patients that were treated with nitrofurantoin 100 mg a day during 10 days, and control group (CG) with 45/46 patients that were not treated. Patients who failed were given a second course of nitrofurantoin. The patients who remained infected were given further treatment (cephalexin). Primary treatment was successful in 85%, 10% more with the second course and 5% were failure treatment. There was a highly significant difference in the incidence of symptomatic bacteriuria (pyelonephritis), premature deliver, prematurity and other perinatal events in patients with persistent infection. Eradication of asymptomatic bacteriuria reduced the risk of pre-term deliveries and lower infant birth weights for gestational age.