Hypercholesterolemic and normal neonates from hypercholesterolemic kindreds were studied for 6 to 30 months. Sixteen of 22 hypercholesterolemic neonates and eight of 11 normal neonates came from families with "monogenic" hyperlipoproteinemia. At 6 or 12 months of age, plasma cholesterol level was greater than 200 mg/100 ml in eight of the 16 neonates with hypercholesterolemia. Four of these eight had cholesterol levels greater than or equal to 290 mg/100 ml at ages 6 to 18 months. On low cholesterol intake, at ages 6 to 12 months, five of seven infants with hypercholesterolemia had cholesterol levels less than 200 mg/100 ml. One of eight normal neonates from families with hypercholesterolemia had cholesterol levels greater than 200 mg/100 ml at ages 6 to 12 months. Neonatal diagnosis of familial hypercholesterolemia provides an opportunity for long-term primary prevention in a group at high genetic risk for premature is chemic heart disease.