Acute pyelonephritis remains a frequent complication of pregnancy. Prevention of the potential complications of this disease requires aggressive in-hospital management. However, the high frequency of positive outpatient cultures following discharge has cast doubt on the adequacy of in-hospital care and, at the same time, has concentrated attention on follow-up care. In a randomized, prospective study, we evaluated the effects of in-hospital management and outpatient nitrofurantoin on subsequent urine cultures. The overall frequency of positive cultures following discharge from the hospital was 38%. However, appropriate antibiotic selection, a negative follow-up in-hospital culture and nitrofurantoin suppression reduced the rate to 8% (P less than .01). Nitrofurantoin did not reduce the rate of positive cultures if antibiotic selection was inappropriate or if the in-hospital follow-up culture was positive. These results suggest that more aggressive management of acute pyelonephritis in pregnancy may be indicated and that suppressive therapy cannot compensate for inappropriate in-hospital management.