Primary hypothyroidism was found to be the cause of hyperlipidemia in 22 patients. The mean age was 46 years, 59% were males, 27% had vascular disease, 14% had xanthomas and 86% had thyroid antibodies. Familial involvement was shown in 3 propositi. All patients were treated with L-thyroxine, 0.05--0.2 mg/day for a mean of 16 months. Combined hyperlipidemia was common (77%), and lipoprotein phenotyping revealed types IIB hyperlipopro-teinemia in 11, IIA in 5, III in 3 and IV in 3 patients. With treatment, normal plasma cholesterol (less than 265 mg/dl) and triglycerides (less than 200 mg/dl) were obtained in 91% and 86%, respectively. The mean maintenance L-thyroxine dose was 0.15 mg/day, but smaller doses often showed marked hypolipidemic effect. The mean +/- S. D. pretreatment fasting plasma cholesterol and triglycerides were 387 +/- 120 and 328 +/-247 mg/dl and on thyroid treatment the mean minimum levels were 205 +/- 46 and 133 +/- 65 mg/dl, respectively (both p values less than 0.005). Hypothyroidism has proved to be a common reversible form of hyperlipidemia. One cardiac patient died and three others had to have their L-thyroxine titrated to prevent angina. Family screening has been of use in case finding for auto-immune disease in 3 families.