To analyze the diagnostic value of various laboratory tests for the confirm
ation of adult-onset glycogen storage disease type II (GSD II), we performe
d a clinical, biochemical, and genetic study of 18 patients with this disea
se. Measurement of acid alpha-glucosidase (GAA) activity in muscle and hist
opathologic analysis of muscle tissue appeared to have no additional value
when GAA activity in leukocytes was clearly deficient. Our study showed tha
t creatine kinase elevation is a sensitive marker of GSD II. A diagnostic p
rotocol is formulated.