A Turkish couple suffering from McArdle disease (myophosphorylase deficiency) and their two sons aged 5 and 9 years, respectively, were studied using 31phosphorus magnetic resonance spectroscopy (31P MRS). During exercise both sons showed the same pathological pattern as their parents. In contrast to healthy volunteers, intracellular pH (pH(i)) as measured by 31P MRS every 10 s during exercise, never fell below the lower 95% confidence limit (6.94) of pH(i) at rest (7.06), but tended to be raised. It is of special interest that the 5-year-old boy was completely asymptomatic, although the enzyme defect seems to be fully expressed at the cellular level.