Point mutations of mitochondrial DNA have been described in the muscle of patients with syndromes of myoclonic epilepsy and ragged red fibres (MERRF) and of mitochondrial encephalopathy with lactic acidosis and stroke-like episodes (MELAS). We have found the MERRF mutation in members of 6 British kindreds; 2 of these had unusual phenotypes but all index patients had myoclonus. The MELAS mutation was detected in 17 patients from 16 families, who had a wide range of clinical features that particularly affected the central nervous system; stroke-like episodes were observed in 10.3 patients with mitochondrial DNA mutations did not have ragged red fibres on muscle biopsy, generally considered to be the morphological hallmark of mitochondrial diseases. In all 6 patients with the MERRF mutation, and 10 of 11 with the MELAS mutation, the genetic defect was easily detected in blood cells as well as muscle (blood samples were not available in 6 patients with MELAS mutations in muscle). Molecular genetic analysis of blood samples represents an inexpensive and reliable screening test for mitochondrial encephalopathies, and use of such techniques could influence diagnosis and genetic counselling in patients with seizure disorders and young-onset stroke.