In order to examine the serotonin metabolism in children with attention deficit disorder (ADD), plasma tryptophan, which is the precursor of serotonin, was measured in 10 children with ADD and 12 normal children. The mean plasma total tryptophan level in the children with ADD was not significantly different from that of the normal children. The mean plasma free tryptophan level in the children with ADD was significantly higher than that in the normal children. There tended to be a positive correlation between the plasma free tryptophan level and the Werry-Weiss-Peters Activity Scale in children with ADD. In other words, the more severe the hyperactivity of ADD, the higher the plasma free tryptophan level. The mean ratio of plasma free to total tryptophan levels in the children with ADD was significantly higher than that in the normal children, which means that the children with ADD showed a high amount of free tryptophan in the total tryptophan level. These results suggest that there might be some disturbance in the tryptophan-serotonin metabolism in the brain of a child with ADD.