This study evaluated the psychiatric characteristics and the behavioral effects of an acute load of phenylalanine on phenylketonuric children. The sample consisted of 13 normal-IQ children who were identified at birth as having phenylketonuria and were subsequently treated with a strict low phenylalanine diet. After baseline measurements, the subjects were loaded with placebo or phenylalanine following a double-blind design. Behavioral and metabolic variables were monitored at hourly intervals for five subsequent hours. The effects of acute loading were measured by changes in a standardized Continuous Performance Task (CPT). Psychiatric symptoms were compiled through a structured psychiatric interview of the child and the parent. Results demonstrated that phenylalanine loading, compared to placebo, caused a nonsignificant decrement in performance on CPT testing. Psychiatric disorders were found in six of the 13 subjects with three currently meeting criteria for attention deficit disorder (ADD). In addition, six other subjects were diagnosed in the past as having ADD using DSM-III criteria.