Myoadenylate deaminase (adenosine monophosphate deaminase—AMPDA) was recently shown to be deficient in a group of patients by use of a histochemical and biochemical method based on the elaboration of ammonia by this enzyme as it deaminates 5′ AMP. We have confirmed the utility of this histochemical method and the existence of persons deficient in AMPDA through the use of an unrelated assay technique. The lack of enzyme activity is not associated with any inhibitory activity in the muscles of patients with this disorder. The clinical diversity of these patients suggests that this lack may represent a normal variant or a subclinical state rather than an actual disease. The occurrence of AMPDA deficiency in both sexes points to possible autosomal inheritance.