In paediatric outpatient departments it is not uncommon for a child to complain of muscle pain and stiffness. These may be labelled as ‘growing pains’, a diagnosis of exclusion when no obvious cause is found. Pain and stiffness occur most frequently at night and are located usually in the leg muscles, and not in the joints. They are intermittent, and tend to occur after the child has had an especially active day, so that a detailed history which notes pain after almost any amount of exercise (as opposed to stress at the site of the pain) will reveal the differences from McArdle's disease. If examination results are entirely normal investigations are to be avoided unless the symptoms persist. If they do, a number of causes may be found, from orthopaedic abnormalities to metabolic disorders. This paper is concerned with one of the latter: glycogenosis type V or McArdle's disease, a rare autosomal recessive condition resulting from muscle phosphorylase deficiency, an enzyme which initiates the breakdown of glycogen in skeletal muscle. Cardiac muscle is not significantly affected. The condition usually presents in the first two decades of life, but can occur in infancy and adulthood.