In this study we present incidence, point prevalence, and mortality figures of mitochondrial encephalomyopathies in a population-based study of children from western Sweden. Through the screening of registers and review of medical records, we identified 32 patients under 16 years of age from the study population who were diagnosed between January 1, 1984, and December 31, 1998. The incidence of mitochondrial encephalomyopathies in preschool children (<6 years of age) was 1 out of 11,000. The preschool incidence of Leigh's syndrome was 1 out of 32,000, and the preschool incidences of both Alper's syndrome and infantile mitochondrial myopathy with cytochrome C oxidase deficiency were 1 out of 51,000. The point prevalence January 1, 1999) of mitochondrial encephalomyopathies in children under 16 years of age was 1 out of 21,000. The median survival for patients with infantile onset was until 12 years of age. We identified 4 cases with mitochondrial DNA point mutations, 2 cases with mitochondrial DNA deletions, and 2 cases with nuclear mutations in the SURF1 gene. We conclude that mitochondrial encephalomyopathies are relatively common neurometabolic disorders in childhood.