The indications for treating patients with subclinical hypothyroidism (normal serum thyroxine and free thyroxine levels, but elevated serum thyrotrophin levels) are poorly defined. In this study, 33 patients with subclinical hypothyroidism were randomly assigned in a double-blind manner to receive placebo or L-thyroxine therapy and were followed for 1 year with thyroid function tests, serum lipid measurements, basal metabolic rate and systolic time interval determinations, and a questionnaire on hypothyroid symptoms. The placebo group showed no changes in thyroid function or peripheral indices of thyroid hormone action. In the thyroxine-treated group, serum lipids and the mean systolic time interval did not change, but the systolic time intervals became normal in the 5 patients with the most abnormal baseline values. Symptoms improved in 8 of 14 patients receiving thyroxine and in 3 of 12 patients receiving placebo (p less than 0.05). L-Thyroxine therapy may be useful for patients with subclinical hypothyroidism with abnormal myocardial contractility or symptoms consistent with mild hypothyroidism, or both.