Twenty-six normal newborns (13 male, 13 female) with normal prenatal histories, no perinatal stress, and normal vaginal deliveries had creatine phosphokinase (CPK) activity and isoenzyme activities assayed in cord blood and in 24-hour postpartum serum. Total CPK activity was high in cord blood when compared with adult control values. Moreover, the total CPK was significantly higher in serum at 24 hours of age compared with cord blood. There was a significant increase in both the skeletal muscle isoenzyme and the cardiac muscle isoenzyme from birth to 24 hours of age. There was a decrease in the brain isoenzyme at 24 hours of age which was not statistically significant. These results were compared with values obtained in a group of 10 neonates with severe cardiac problems. Three of the ill neonates had significant elevation of total serum CPK and the skeletal muscle isoenzyme when compared with the normal newborns. There were no significant differences between the normal infants and the ill neonates for the cardiac isoenzyme and the brain isoenzyme. These data suggest that caution should be used in the diagnosis of certain neonatal cardiac syndromes based on serum CPK levels and isoenzyme alone.