McArdle's syndrome. Fine structural changes in muscle.

Two cases of McArdle's syndrome are reported. One is a "classical" example; the other is unusual because of the in vitro presence of muscle phosphorylase activity. In the latter case, the electronmicroscopic investigation confirmed the diagnosis. The fine structural changes characteristic of this disease are summarized and it is concluded that histochemical studies alone are insufficient to exclude the diagnosis of McArdl's myopathy.