The T8993G mutation in the mitochondrial DNA adenosine triphosphatase 6 gene represents an important cause of maternally inherited Leigh's syndrome. Reported are the clinical findings and mutational loads in three Portuguese T8993G pedigrees. Polymerase chain reaction-restriction fragment length polymorphism analyses demonstrated the T8993G mutation in a high percentage of tissues from all patients (97% +/- 2.3%), but it was less abundant in the blood from 14 maternal relatives. The disease progressed severely in the probands but did not have the fatal course reported by others. To test whether this prolonged course was related to the presence of a specific, disease-associated haplogroup the origin of the mutational event in Portugal was traced. Haplotype investigation revealed an independent occurrence of the mutation in the three probands. These analyses represent the first molecular characterization of Portuguese patients with Leigh's syndrome.