Phosphoglucomutase 1 deficiency has recently been reported as a novel disease that belongs to two different classes of metabolic disorders, congenital disorders of glycosylation (CDG) and glycogen storage diseases.This paper focuses on previously reported siblings with short stature, hypothyroidism, increased transaminases, and, in one of them, dilated cardiomyopathy (DCM). An intronic point mutation in the PGM1-gene (c.1145-222 G>T) leads to a complex alternative splicing pattern and to almost complete absence of PGM1 activity.Exercise-induced muscle fatigue, chest pain, and rhabdomyolysis persisted into adulthood. Fainting occurred during the first minutes of strong exercise due to glucose depletion and serum heart troponin was increased. A second wind phenomenon with an improvement in exercise capacity after some minutes of training was observed. Regular aerobic training improved fitness and helped to avoid acute damage. DCM improved during therapy.Glycosylation deficiency was most prominent in childhood. Glycosylation improved with age and further improved with oral galactose supplementation even in adulthood. Optimal improvement of glycosylation-dependent phenotypes should be achieved by early and permanent galactose treatment.However, in case of mutations in ZASP, DCM can develop as a consequence of impaired binding of PGM1 to the heart-specific isoform of ZASP, independently of overall glycosylation efficiency. Thus, even if mutations in PGM1 impair the function of the ZASP-PGM1 complex, supplementation of galactose cannot be expected to restore that function. Therefore, knowledge of PGM1 deficiency in a patient should prompt surveillance of early signs of DCM and specific treatment if necessary.