An infant with hypotonia and recurrent apneic spells died with a diagnosis of pyruvate dehydrogenase deficiency and showed typical pathological changes of Leigh's syndrome at postmortem. Despite the prominence of symptoms suggesting dysfunction of brainstem respiratory centers during life, lesions were not found in the upper medulla. However, quantitative morphometric analysis demonstrated abnormal capillary hyperplasia in the region including and between the nucleus ambiguus and nucleus tractus solitarius. There was an average area of 8.0 +/- 2.5 x 10(6) mm2 occupied by capillaries per 0.75 mm2 field in the patient's brainstem, compared with 4.6 +/- 1.6 x 10(6) mm2 and 5.5 +/- 1.4 x 10(6) mm2 in two age-matched controls (p < 0.01). We speculate that capillary hyperplasia is a pathological marker of chronically impaired oxidative metabolism in the central nervous system in metabolic disease.