Eleven children from eight families with autosomal dominant polycystic kidney disease who were diagnosed in utero (6 children) or in the first year of life (5 children) are reported here. Four children were evaluated for symptoms and three because of a sibling with very early onset disease. In three children, abnormal kidneys were found incidentally on a pregnancy screening ultrasound, and in only one child, the diagnosis was made by an ultrasound specifically directed at detecting polycystic kidney disease. Females were disproportionately represented among both the affected parents and offspring. Eight of the children were girls, and all affected parents were mothers. In three families, the parent's diagnosis was established only after the birth of the affected child. In two of these and in one other family, the mother's disease appeared to be the result of a new mutation. The most consistent renal ultrasonographic findings in the children were enlargement and increased echogenicity. On follow-up over 3 to 15 yr (mean, 6.8 yr) two children had ESRD and eight children had normal or nearly normal renal function as assessed by creatinine clearance. Renal concentrating ability was reduced in four children in whom it was measured. All children had bilateral renal cysts on follow-up, and nine children were hypertensive. Possible risk factors for early-onset disease identified in this study were an affected mother, an affected sibling, and an apparent parental new mutation. Symptoms and complications occurred frequently, but outcome was better than reported previously.