Ten of 16 boys with Duchenne muscular dystrophy and one of 11 patients with limbgirdle muscular dystrophy were found to have increased content of fibrin split products in serum. In both diseases, an increase in the content of fibrin split products was found only in those patients in whom the creatine phosphokinase activity was greater than 1,000 international units, suggesting that the activation of fibrinolysis in muscular dystrophy results from leakage of plasminogen-activator from diseased muscle.