Serum thyroid hormones and antithyroid autoantibodies (AAB) were assayed in 87 randomly selected hypercholesterolemic persons compared to 80 controls with normal serum total cholesterol (TC). Of the 87 hypercholesterolemic persons 22 (25%) had positive AAB compared to 5 (6%) controls. Furthermore, 8 of the hypercholesterolemic patients had a serum TSH level above 5 mU/l, i.e. the had subclinical hypothyroidism, not diagnosed before, whereas thyroid function was normal in all normocholesterolemic persons. The new and unexpected finding was that the hypercholesterolemic persons had on average a significantly higher serum TSH than the controls, and this was true even when persons with positive AAB were excluded. There was a significant correlation between TC and serum TSH. It is concluded that hypothyroidism may not be an all-or-none phenomenon, and that many hypercholesterolemic persons with thyroid tests within the conventional normal range may have a slight impairment of their thyroid function.