McArdle's disease is a hereditary, metabolic myopathy characterized by weakness and muscle cramps after exercise, appearing mostly in the second or third decade of life. Due to myophosphorylase deficiency in skeletal muscle, glycogen cannot be used and deposited in the sarcolemmal spaces, leading to lack of endurance to sustained work. The ischemic exercise test is a screening procedure for muscle energy disorders, and the diagnosis is confirmed by reduced enzyme activity in muscle biopsy. In this report, a family with one child having enzyme assay-proven McArdle's disease and two other children demonstrating a positive ischemic exercise test is presented.