Two sisters with chronic progressive external ophthalmoplegia (CPEO) and their in all 7 healthy children were investigated. Both ophthalmoplegic patients had histopathological changes typical of mitochondrial myopathy. The same type of muscular pathology was also found among the healthy children. The most common muscular changes were subsarcolemmal accumulation of pathological mitochondria, including vacuoles, abnormal cristae and sometimes also inclusion bodies. Biochemical studies showed partial complex III deficiency, with low succinate-cytochrome c reductase activity in 1 of the ophthalmoplegic patients. These findings suggest that CPEO is a slowly progressive muscle disease, starting early in life. The widespread occurrence among the children may indicate maternal inheritance.