Serum lipids and the cholesterol concentrations in the high density lipoprotein (HDL) fractions were measured in patients receiving long-term corticosteroid treatment for connective tissue disorders and asthma. Patients who were not receiving corticosteroid treatment had blood lipid levels which did not differ from those of healthy people. However, female (but not male) patients who had received prednisolone for a mean period of 3.1 years had a significant elevation in total cholesterol and a large decrease in HDL cholesterol. It seems possible that high levels of corticosteroids may increase the incidence of premenopausal ischaemic heart disease in females.