We present a patient with external ophthalmoplegia, bilateral ptosis, progressive muscle weakness with "ragged-red fibres" and mental retardation. Mitochondrial DNA analysis by Southern blot revealed heteroplasmy in muscle for a 7.4 kb deletion. In white blood cells, the deletion was only detectable by PCR. There was no evidence for duplications, nor for multiple deletions in the proband or siblings. PCR analysis did not reveal the presence of a mitochondrial DNA defect in the parents and siblings. Thus, there is no experimental support for a maternally inherited mitochondrial DNA deletion. We consider this a sporadic case with a de novo deletion. Diabetes and complaints of fatigue, also seen in this family, are probably coincidental. Mental retardation has been reported occasionally in patients with mitochondrial deletions, but is not common.