Serum creatine phosphokinase was assayed by a spectrophotometric method in relatives of three boys with the Aran-Duchenne type of progressive muscular dystrophy. All patients had markedly elevated enzyme concentrations. Four additional cases were discovered during the investigation. The highest serum creatine phosphokinase value was observed in an asymptomatic 13-month-old child. Eight out of nine clinical conductors (mothers and grandmothers) had elevated values and so had approximately half of their daughters. A relatively high incidence of false positive results was obtained. These could be shown to be due to muscular trauma and exercise which leads to marked increases in serum creatine phosphokinase. The possibility is considered that the defect in membrane permeability may be present at birth and may play a role in the development of the disease.