Nineteen patients meeting the criteria for attention deficit disorder, residual type (adult hyperactivity), were given a 2-week double-blind crossover of DL-phenylalanine versus placebo. Thirteen subjects completed the study; the mean global rating of improvement approached significance as compared with placebo. A significant improvement was noted on mood and mood lability. The phenylalanine responders were then continued on open drug, but lost all positive benefits within 3 months. A later open trial of L-phenylalanine produced no clinical effect.