In situ hybridization to mitochondrial ribosomal RNA (rRNA) has been used to study the distribution of mitochondria in paraffin-embedded autopsy brain tissue from two patients with MELAS (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes) and other organs from one of the patients. Comparison of in situ hybridization and electron microscopic findings in an antemortem biopsy specimen of pylorus from the latter patient showed a close correspondence between the distribution of hybridization signal on light microscopy and of mitochondria in ultrathin sections. Strong hybridization signal was present over smooth muscle fibres of the muscularis externa, which contained abnormal accumulations of mitochondria on electron microscopy. Hybridization to sections of skeletal muscle confirmed previous reports of 'ragged-red' fibres in this disorder and of mitochondrial accumulations in the walls of intramuscular blood vessels. To try to elucidate the role of vessel wall accumulation of mitochondria in the genesis of the stroke-like lesions, the distribution of mitochondrial rRNA was assessed in sections of brain from both of the cases of MFLAS and several cases of atherothrombotic cerebrovascular disease. Blood vessels in and adjacent to the cerebral lesions of MELAS showed strong hybridization signal with the mitochondrial probes, as was also seen in infarcts of various ages in the control brains. Only weak signal was present in the walls of blood vessels distant from the lesions, in both MELAS and control brains. These findings suggest that mitochondria accumulate in vascular endothelium and tunica media as a normal response to cerebral infarction or ischaemia. The accumulation of mitochondria in the cerebral lesions of MELAS may, at least in part, be a reaction to the destructive effects of the underlying metabolic dysfunction.