Serum coenzyme Q10 (CoQ10) levels were measured at rest and during incremental exercise in 21 patients with chronic obstructive pulmonary disease (COPD) and 9 patients with idiopathic pulmonary fibrosis (IPF). The mean serum CoQ10 levels at rest in patients with COPD and IPF were 0.56 +/- 0.20 and 0.45 +/- 0.16 microgram/ml, respectively. In both groups these levels were decreased compared with those of healthy subjects. In the patients with COPD, CoQ10 levels were significantly correlated with body weight, however, there was no correlation between CoQ10 levels and ventilatory function, PaO2, VO2/kg at rest, or maximal VO2. In eight of nine patients whose PaO2 at rest was lower than 75 torr, serum CoQ10 levels were lower than 0.5 microgram/ml. We studied the effects of the oral administration of CoQ10 at 90 mg/day for 8 weeks on pulmonary function and exercise performance in eight patients with COPD. Serum CoQ10 levels were significantly elevated in association with an improvement in hypoxemia at rest, whereas pulmonary function was unaltered. Oxygen consumption during exercise was not changed, whereas PaO2 was significantly improved, and heart rate was significantly decreased compared with the results obtained at an identical workload at baseline. Furthermore, lactate production was suppressed during the anaerobic exercise stage after CoQ10 administration, and exercise performance tended to increase. These data suggested that CoQ10 has favorable effects on muscular energy metabolism in patients with chronic lung diseases who have hypoxemia at rest and/or during exercise.