Recent studies with normal monozygotic and dizygotic twins have established that serum creatine phosphokinase (CPK) activity in man is under genetic control (Meltzer, Dorus, and Davis, in preparation). This is of interest because of the reports that serum CPK activity is increased in many acutely psychotic patients, provided that they are studied in the first days after the onset of gross psychotic symptoms and throughout hospitalization.1-12 The increases range from slightly above the 95% upper limits of normal to 50-75 times the limits for each race-sex group. Furthermore, 25%-30% of the first degree relatives of psychotic patients have slight but persistent increases in serum CPK activity.2,3,7 These increases occur significantly more frequently in relatives of psychotic patients who have themselves had increases in serum CPK activity (Meltzer and Moretti, in preparation). Prior to the studies with normal twins which established that serum CPK activity is under some genetic control, we had the opportunity to study serum CPK activity in twins discordant for schizophrenia. These results will be reported here.