In 16 children heterozygous for familial hypercholesterolemia, two- to three-year therapy with diet and cholestyramine resin (16 gm/day) was assessed in terms of effectiveness, practicality, and safety. All 16 children had previously taken a low-cholesterol (less than 300 mg/day), polyunsaturate-rich (P/S ratio, 1.5:1) diet and choeltyramine resin (12 gm/day) for 12 months. In this study, the cholestyramine resin dose was increased to 16 gm/day, and follow-up was maintained through months 13 through 18, 19 through 24, 25 through 30, and 31 through 36. Eleven children had good drug adherence (four packs of cholestyramine per day) and five children had fair adherence (two to three packs per day). Plasma total cholesterol and low-density lipoprotein (LDL) cholesterol levels were not significantly lowered on the drug-plus-diet regimen as compared to diet alone in five children with fair drug adherence. For children with good drug adherence, mean plasma cholesterol level was lowered below levels achieved on diet alone by 13% (months 13 through 18), 12% (months 19 through 24), 12% (months 25 through 30), and 11% (months 31 through 36) (P less than .05). Reduction in plasma cholesterol level was no greater with 16 than with 12 gm of cholestyramine per day. There were no group changes in mean plasma triglyceride levels. Cholestyramine resin, when added to diet and maintained for two to three years effects a significant reduction in total and LDL cholesterol levels in about 60% of children heterozygous for familial hypercholesterolemia. Continued reinforcement of both diet and drug adherence is necessary in the face of gradual increments in plasma cholesterol level with time.