The clinical course of MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, stroke-like episodes) is described in five cases. The age of onset was 9.2 years on average. Cerebral symptoms occurred at an age of 24.6 years on average. The diagnosis was only made later. Three patients died during the follow-up period. The mean age at death was 33.3 years, the average duration of the disease, 19.6 years. All patients suffered from progressive hearing loss, multiple strokes, and complex-partial and generalized seizures. They all had short stature, normal early development and 'ragged red fibers' in the muscle biopsy. Imaging studies (CT and MRI) showed bilateral calcification of basal ganglia and multiple strokes. One patient, admitted with epileptic status as initial symptom, showed lymphocytic pleocytosis in the cerebrospinal fluid; thus, in her case the differential diagnosis included encephalitis. Four patients were studied by dynamic endocrinological testing. Endocrinological disturbances were detected in three patients. Two of them suffered from hypothalamic-pituitary failure and one patient exhibited hyperprolactinemia. Endocrinological disturbances were considered an expression of the underlying cytopathy. Cerebral symptoms occurred rather late in the course of the disease and indicated poor prognosis.