McArdle's disease is caused by a mutation in the PYGM gene, causing a muscle myophosphorylase deficiency, altering the release of glucose-1-P from glycogen. It usually manifests itself in childhood with early and excessive tiredness, myalgias, cramps and contractures or rhabdomyolysis, although it is not usually diagnosed until adulthood. Creatine kinase increases sharply during exercise. Four pediatric patients are presented, the pathophysiology is summarized, and a diagnostic algorithm is proposed. Ages between 6 and 14 years, the anamnesis, physical examination, biochemistry, elec-tro-myogram, ischemia test and genetic study are described. Muscle biopsy in a single patient. The algorithm was developed from the ischemia test. In the three men, myalgias appeared after finishing each sports session. Phenomenon 'second wind' in one case. Ischemia test without lactate elevation and marked ammonia elevation in all. Only one muscle biopsy with glycogen deposits and absence of myophosphorylase. PYGM gene with homozygous mutations in all. Dietary treatment attenuated their symptoms during aerobic exercises. The ischemia test was very useful to demonstrate a dysfunction in anaerobic glycolysis. It is worth noting that oral glucose supplementation is very useful in McArdle disease, but is contraindicated in all six defects of anaerobic glycolysis. The algorithm also allows targeting the defect of 20 metabolic or structural myopathies, which are summarized.