Multicystic dysplastic kidneys (MCD) were found in 17 of 114 neonates with prenatal diagnosis of urinary tract malformations. Distribution of side and of sex was not different. Contralateral malformations were present in 3 infants. One of them with contralateral renal dysplasia and cardiac malformation died at the age of 4 weeks. All other children so far have a normal renal function. Three neonates presented with a palpable abdominal mass, 2 infants had urinary infections during the first year of life. Two neonates had obstruction of the contralateral kidney caused by the giant MCD which relieved after nephrectomy. Hypertension or development of malignancy were not noted. Nephrectomy was performed in 10 infants at the mean age of 3.2 months. Six infants had conservative treatment and a complete regression was noted in 5 of them within a period of 8 to 18 months. Prenatal diagnosis of MCD enables early recognition of contralateral urinary malformations and of problems caused by the MCD itself. Conservative treatment is recommended in all asymptomatic patients. Studies of the natural history may show that regression of MCD is the rule and could account for many cases with apparent unilateral renal agenesis.