Myoadenylate deaminase (MAD) is the rate-limiting enzyme in the purine nucleotide cycle which is biochemically linked to glycolysis and the citric cycle and thereby providing energy during intense muscular activity. In muscle fibers, myoadenylate deaminase operates at considerably higher activity levels than in other organs. First detected using enzyme-histochemical methods, it now appears that deficiency of myoadenylate deaminase is one of the most frequent enzyme defects in muscle. The primary defect may occur as an isolated nosological entity or not infrequently it is also associated with a large spectrum of different neuromuscular conditions. It seems to be the primary unassociated MAD deficiency that has recently become amenable to successful treatment with D-ribose in high doses. Secondary MAD deficiency may occur in muscle fibers and muscles that have undergone structural damage as seen, for instance, in polymyositis, muscular dystrophy, and denervation. The wealth of biochemical, morphological, and clinical data that has accumulated since the discovery of MAD deficiency during the past decade provides nosological significance of MAD deficiency as a real entity.