A 46-year-old white female was admitted for decompensated cor pulmonale (CP). It had not interfered with her daily activities and she had not experienced shortness of breath, fatigue or muscle weakness prior to the onset of right heart failure. A thorough investigation revealed severe generalized muscle weakness with restrictive chest bellows disease and secondary CP (mean pressure in the pulmonary artery 60 mm Hg). After having refused respiratory support the patient died a few days after admission. The muscle biopsy was consistent with adult-onset acid-maltase deficiency. This is a rare case of metabolic myopathy presenting as decompensated CP without previous symptoms of muscle weakness. This condition can easily be treated with nocturnal ventilatory support, improving the quality and length of life.