Twenty-five hypercholesterolaemic patients from three centres in the UK were investigated in an open study of the efficacy and side effects of niceritrol. Five patients dropped out of the study at an early stage and had insufficient data for analysis. There were 13 males and 7 females (mean age 49.2 years, range 18-69). Fourteen patients had heterozygous familial hypercholesterolaemia, and six polygenic hypercholesterolaemia. Niceritrol was started at a dose of 750 mg/day and this was increased at weekly intervals over 4 weeks to the maximum tolerated dosage up to 3 g/day. This was then maintained for a further 8 weeks. There were statistically significant decreases in total plasma cholesterol, total triglyceride, LDL cholesterol and VLDL triglyceride; HDL cholesterol remained unchanged after 12 weeks of treatment (Wilcoxon matched pairs, signed ranks test). The 14 patients with familial hypercholesterolaemia showed a 13.9% fall in total cholesterol and a 19.8% fall in LDL cholesterol. All patients reported flushing and some had gastrointestinal symptoms but 19 would have been prepared to continue with the therapy at doses up to 3 g/day. Thus niceritrol has been found to be beneficial in the treatment of both familial and polygenic hypercholesterolaemia.