Effects of a cholesterol- and saturate-poor, polyunsaturate-rich diet on plasma total and low-density lipoprotein (LDL) cholesterol levels were assessed in 23 children between the ages of 2 and 7 who were heterozygous for well-documented familial hypercholesterolemia. Sixteen of the 23 children, whose mean age at inception of diet was 4.8 years, sustained 10.5% and 11.3% reductions in total and LDL cholesterol levels after six months on diet (P less than .001). Total and LDL cholesterol levels were normal in only one of these children after six months. After one year on diet, three of 11 children had normal values for total and LDL cholesterol. Six of the 23 children, all 2 years old, had been previously maintained on low-cholesterol diets since age 1 or earlier, and three had normal cholesterol levels at the initial sampling for this study. After one year of follow-up in the study, the plasma cholesterol level was normal in five of these six children. Dietary therapy in the 2- to 7-year-old children heterozygous for familial hypercholesterolemia provided mean overall reduction of total and Ldl cholesterol levels between 6% and 15%, and appeared to be the most effective when instituted in children at ages 1 or 2.