The optimal treatment, surgical or conservative, for multicystic dysplastic kidney (MDK) is still undetermined. We analyzed prospectively the evolution of 12 children with MDK who were managed conservatively. After a mean follow-up of 32 months, the only complication found was an episode of urinary tract infection in 3 patients. Complete involution of MDK was observed in 9 patients (75%), the mean period for sonographical disappearance being 16 months. Infants with MDK should be closely monitored and nephrectomy performed when MDK compresses neighbor organs or hypertension develops. There is no agreement on the best therapeutic approach in patients in whom MDK involution has not occurred after a prudential waiting time still not determined.