Absence of AMP-deaminase was demonstrated by histochemical and biochemical methods in a muscle biopsy of a 25-year-old woman with facial and limb girdle myopathy. Venous ammonia failed to rise after ischaemic exercise. This patient further contributes to the variety of clinical pictures associated with AMP-deaminase deficiency. Whereas AMP-deaminase has been shown to play an essential role in the regulation of adenine nucleotide metabolism in the liver, its physiological function in muscle remains uncertain.