McArdle disease was diagnosed in a 25-year-old female with a history o
f muscle weakness and painful muscle cramps after exercise since adole
scence. Blood tests after exercise found evidence of myolysis without
the normal exercise-related elevation in lactic acid. Elevated glycoge
n content was found in biopsied muscle tissue. No myophosphorylase act
ivity was detected. The literature on this probably underdiagnosed dis
ease is reviewed.