Urinary excretion of carnitine and serum concentrations of carnitine, triglyceride, and free fatty acids were measured in 54 hyperthyroid and 13 hypothyroid patients, and the results were compared with those of normal subjects. In hyperthyroid patients urinary excretion of carnitine was highly increased above that of the control subjects. On adequate treatment with antithyroid drug, carnitine excretion was reduced to the normal range, and serum lipids changed in parallel. In contrast, carnitine excretion was markedly reduced in hypothyroid patients. After substitution therapy with thyroid hormones the excretion increased in these patients. This change was associated with a marked reduction of serum triglyceride. There was an inverse correlation between urinary excretion of carnitine and serum triglyceride concentration. Carnitine excretion was significantly correlated with serum thyroxine concentration in hyper- and hypothyroid patients. The results suggest that thyroid hormones play an important role in carnitine metabolism, which in turn influences serum triglyceride metabolism.