Catechol-O-Methyltransferase (COMT) activity was studied in three groups of institutionalized children: (1) a group of schizophrenic children; (2) a heterogeneous group of chronic psychotic children characterized by severe symptomatology and onset before 5 years of age; (3) a group of acting-out but nonpsychotic children. Erythrocyte COMT activity was found to be significantly lower among the schizophrenic subjects in contrast to the greater activity in both the other groups--the nonpsychotic and chronic psychotic children. The difference in COMT activity between psychotic groups appeared to be related to diagnosis and age of onset of disorder. Generality of findings is limited by the small sample size (N = 42) and by the difficulties inherent in the diagnosis of severe mental disorder in children. However, this preliminary study suggests that enzymatic activity may be associated with the development of schizophrenia in children.