Previous studies have suggested increased protein catabolism and altered muscle metabolism in hyperthyroid patients. In this experiment we investigated parameters of protein and leucine metabolism before and after treatment of hyperthyroidism. While confined in a metabolic ward, patients' daily caloric intake was based on the resting energy expenditure and an allowance for 16 hours of light physical activity. We found no significant difference in plasma aminogram and urinary 3-methylhistidine excretion (an index of protein catabolism) before and after treatment. On the other hand, hyperthyroidism appeared to increase the rates of oxidation, turnover, and plasma clearance of endogenous leucine. However, only the 60% increase in the rate of leucine oxidation was statistically significant. We conclude that in spite of increased catabolism, basal levels of branched-chain amino acids are well maintained in plasma of hyperthyroid patients consuming a diet that compensates for their hypermetabolic state.