We investigated the phenomenon of second wind in four patients with McArdle's disease: a brother and sister (aged 4 and 12 years respectively) and two unrelated patients, a boy of 14 and a 17-year-old girl. We also studied the siblings' healthy 6-year-old sister. Each patient performed a 15-min exercise test at a constant workload and a subsequent graded exercise test until exhaustion. Overall the healthy girl and the youngest McArdle's patient, the 4-year-old boy, did not show a second wind phenomenon. Further, the peak cardio-respiratory capacity of the young McArdle's boy was normal for his age (32.3 mL 02/kg/min) and he did not report any function limitations during physical education classes.