Sixteen patients with hypothyroidism have had their lipid status assessed before and during replacement therapy. More than 60% had hypercholesterolaemia and more than 60% had hypertriglyceridaemia. Significant reductions in plasma cholesterol, but not in plasma triglyceride, were seen during replacement therapy. A high cholesterol: triglyceride ratio was observed in VLDL and this relationship tended back to normal during treatment. This raises the possibility that in hypothyroidism, as in Type III hyperlipoproteinaemia, an abnormality in VLDL conversion to LDL is present.