Cholestyramine in a mean dosage of 0-6 g/kg/day has been given to 18 children with familial hypercholesterolaemia for between one and two and a half years. With prolonged treatment folate deficiency occurred, as evidenced by a fall in the mean serum folate concentration from 7-7 ng/ml before treatment to 4-4 ng/ml for patients on treatment for over one year; a corresponding lowering of red cell folate was also seen. Oral folic acid 5 mg daily overcame this depletion, and should be given to all patients on long-term anion exchange resins. Prothrombin time has remained normal in all patients; there has been a significant decrease in the mean serum concentrations of vitamins A and E and of inorganic phosphorus over the first two years of treatment, although values remain within the normal range. The routine administration of fat-soluble vitamins appears unnecessary but it is prudent to measure prothrombin time and serum vitamins A and E at intervals. In children who were having a normal intake of dietary fat five out of seven tested had faecal fat of over 5 g/day while on cholestyramine. No chold has developed diarrhoea, and growth has been normal. The concentrations of serum iron, vitamin B12, plasma calcium, and protein did not change significantly in any patient.