We used a standardized bicycle ergometry protocol with a stepwise increasing workload (30-100 W) to evaluate various metabolic factors for the diagnosis and metabolic monitoring of mitochondrial encephalomyopathies. All patients (n = 9) showed pathological venous lactate/pyruvate (L/P) ratios, which normalized in three patients after 6 months of coenzyme Q10 (CoQ) therapy. Thus, the L/P ratio proved to be the clinically most useful parameter in the evaluation and monitoring of mitochondrial diseases, showing higher sensitivity than lactate measurements only. CoQ may exert a favourable effect in some patients with mitochondrial diseases.