Wolman's disease is a rare autosomal recessive disease due to lysosomal acid lipase complete deficiency (McKusick 27.800). Prenatal diagnosis is based on safe chorionic villus sampling procedures. We test acid lipase activity in cultured chorionic villus cells, selected from a biopsy performed during the 8th week of pregnancy. We now report the first prenatal diagnosis of heterozygosity for Wolman's disease during the first trimester of pregnancy. Reduced acid lipase activity was shown in the chorionic villi cells using a natural substrate (Cholesterol 14C oleate). The diagnosis was confirmed by the demonstration of reduced acid lipase activity in cultured amniotic cells and in the newborn lymphocytes. Early prenatal diagnosis in pregnancies at risk for lysosomal storage diseases is possible when enzyme activity levels in chorionic villi are similar to those in cultured amniotic cells and in infant cells.