Ten adult patients complained of exercise intolerance; five of them had cramps and three had recurrent myoglobinuria. Resting serum CK was increased in five. Muscle biopsies showed phosphorylase b kinase (PbK) deficiency, whereas the activities of other enzymes of carbohydrate metabolism were normal. None of the patients exhibited symptoms indicative of liver PbK deficiency. Thus, these patients are new additions to a class of PbK glycogen storage disease characterized by enzyme deficiency in muscle but not liver. Family histories were consistent with autosomal recessive transmission. Monoclonal antibodies specific for the beta and gamma subunits of PbK cross-reacted differentially with muscle biopsies from three of these patients, suggesting that this phenotype of PbK deficiency is biochemically heterogeneous.