Systemic cytochrome oxidase deficiency presenting as Leigh syndrome is a well‐defined biochemical entity. Although the enzyme defect is demonstrable in all tissues, clinical abnormalities are restricted to the central nervous system. Biochemical studies comparing rates of synthesis of cytochrome oxidase subunits with the steady‐state levels of immunoreactive protein in the mitochondrial inner membrane suggest a defect in assembly or stability of the complex. Family studies suggest that the disease is inherited as an autosomal recessive and somatic cell genetic studies directly implicate nuclear genes. As there are likely to be a number of different nuclear genes involved in the synthesis, assembly and stability of the cytochrome oxidase complex, we have fused patient fibroblasts and analysed the heterokaryons for complementation of the enzyme defect in an attempt to define the extent of genetic heterogeneity in this condition. So far, three complementation groups have been defined, although the majority of patients fall into a single group.