Leber's hereditary optic neuropathy (LHON) is a maternally inherited disorder characterized by central vision loss in Young adults. The majority of LHON cases around the world are associated with mutations in the mitochondrial 1, genome at nucleotide positions (np) 3460, 11,778, and 14,484. Usually, these three mutations are screened in suspected LHON patients. The result is important not only in respect to the diagnosis but also as different LHON mutations lead to variations in expression, severity. and recovery of the disease. There are, however, a significant number of patients without any of these primary mutations. In these Situations, genetic counselling of a patient and his family can be difficult. We sequenced the complete mitochondrial DNA (mtDNA) in 14 LHON patients with the typical clinical features but without a primary mtDNA Mutation to evaluate the potential of extensive Mutation screening for clinical purposes. Our results suggest to include the Mutation at up 15,257 in a routine screening as well as the ND6 gene, a hot spot for LHON mutations. Screening for the secondary LHON mutations at up 4216 and up 13,708 may also help in making the diagnosis of LHON as these seem to modify the expression of LHON mutations. Although they do not allow to prove the clinical diagnosis, their presence increases the probability of LHON. Sequencing the complete mitochondrial genome can reveal novel and known rare disease causing mutations. However, considering the effort it adds little value for routine screening. (C) 2002 Elsevier Science (USA). All rights reserved.