Mitochondrial respiratory chain dysfunction has been studied by two complementary methods using cultured skin fibroblasts from five patients with muscular cytochrome c oxidase (complex IV) deficiency: first, a screening test measuring lactate to pyruvate ratio (L/P) after supplementation of cultured cells; secondly, measurement of complex IV activity in whole cells. Respiratory chain defect (increased L/P ratio with decreased complex IV activity) was expressed in fibroblasts of four of the five patients. Our results show that skin fibroblasts can be helpful for biochemical diagnosis of mitochondrial respiratory chain defects.