Longitudinal studies of the effects of a cholesterol-free diet and a less rigid 300 mg/day low cholesterol diet, both with a polyunsaturated to saturated fatty acid ratio of 1.8/1, were carried out preconception, during gestation, and postpartum in a women heterozygous for familial hypercholesterolemia. On the cholesterol-free diet, during weeks 8-14 of gestation, plasma cholesterol was lowered 25% (from 310 to 230mg/dl), and plasma low density lipoprotein cholesterol (C-LDL) (from 240 to 160 mg/dl), 33%. The 25% reduction in plasma cholesterol was slightly more than previously reported decrements of 19% in 14 normal women during pregnancy, also receiving a cholesterol-free diet. The 300 mg cholesterol diet was not as hypocholesterolemic as the cholesterol-free diet. Its maintenance throughout gestation limited the within-pregnancy increments of total plasma cholesterol and C-LDL to 21% (352-426 mg/dl) and 14% (286-326 mg/dl) respectively. Both the cholesterol-free and the 300 mg cholesterol diet were well tolerated, and should be nutritionally adequate for pregnant women, since they contain more than the recommended amounts of high quality protein, vitamins, minerals, and calories for pregnant women. Cholesterol restricted diets during pregnancy in familial hypercholesterolemics should reduce the physiologic hypercholesterolemia of pregnancy, and potentially reduce the increased risk of coronary heart disease relative to the degree and duration of elevations of total and LDL cholesterol.