Hypothyroidism is a classical cause of hypercholesterolaemia. As we sometimes met patients with hyperlipidaemia whose thyroid state was unknown, we carried out a prospective study aimed at evaluating the frequency of hypothyroidism in a population of 1210 hyperlipidemic patients who were referred to our out-patient clinic at the La Pitié Hospital, Paris, for metabolic and cardiovascular assessment. The proportion of subjects with high thyroid-stimulating hormone (TSH) levels was 12, 56 percent, which is distinctly higher than the figures reported in previous studies in patients who were not selected for lipid abnormalities. Among those with high TSH levels, 16 had overt hypothyroidism with a low free T4 level. Analysis of lipid parameters showed that hypertriglycidaemia was frequent and did not confirm the hyperHDLaemia classically observed in hypothyroidic populations. We conclude that screening for hypothyroidism by measuring TSH values is of particular importance in patients with hyperlipidaemia, especially in the group of women over 50 years of age.