1. To investigate the effects of starvation, elective surgery, accidental injury and other clinical conditions on the metabolism of branched-chain amino acids in man, we have measured the basal concentration of leucine and the removal of metabolic effects of infused L-leucine. 2. The blood concentration of leucine as significantly increased by surgery, starvation and accidental injury, and decreased in cirrhosis. It tended to increase in diabetes and was unaffected by muscular dystrophy. 3. The half-life of infused leucine was nearly doubled by 4 days of complete starvation, unaltered by surgery and decreased by severe accidental injury, Infusion with Intralipid, which increased free fatty acid and ketone-body concentrations, had no effect on the removal of a leucine load. The clearance rate of infused leucine was reduced in diabetes and muscular dystrophy and increased in cirrhosis. 4. The effects of infused leucine on blood glucose and ketone bodies differed according to the groups studied. 5. Since the traumatized patients were given sufficient energy and nitrogen and disposed of a leucine load at a different rate from the starved patients, the causes of the increase in blood concentration of leucine in these two conditions are different.