Objective: Cytochrome c oxidase (COX) deficiency presents with severe impairment of brain, muscle or heart. Prenatal diagnosis in affected families is difficult because the disease may be caused by mutations in nuclear or mtDNA. This study shows the results of prenatal diagnosis in two families where the first child died because of a generalised COX defect. In both cases the low activity of COX was accompanied by a low content of the enzyme. Subjects: In the first family the amniocentesis was performed during the second pregnancy and cultured amniocytes showed a marked decrease of COX activity and ATP production. Based on decision of the parents the pregnancy was terminated. Analysis of the foetal tissues confirmed a generalised COX defect. In the second family the nuclear origin of the COX defect was found using transmitochondrial cybrids derived from COX-deficient fibroblasts of the affected child. In the successive pregnancy with dizygotic twins a combined amniocentesis and chorionic villi bio- psy has been performed. Prenatal diagnosis was based in both foetuses on three independent approaches. COX activity, the ATP production and protein content of COX complex was measured in cultivated foetal cells. The results of all investigations excluded a putative COX defect and both children are healthy at the age of 2 and half years. Conclusion: Prenatal diagnosis of COX disorders is available in families with the generalised form of the disease based on a nuclear origin of COX deficiency. Three independent approaches to characterise COX at a functional, enzymatic and protein level may be used.

        Key words: cytochrome c oxidase, transmitochondrial cybrids, mtDNA