A 2.5-year-old boy with bilateral striatal lesions is reported. Using polymerase chain reaction-single-strand conformation polymorphism analysis and direct DNA sequencing, a novel point mutation (T to C) at nucleotide 8851 of the mitochondrial DNA (mtDNA) was identified. This mutation changes a highly conserved tryptophan to arginine in subunit 6 of the mtATPase gene. The mutation was nearly homoplasmic and maternally inherited. This is the first published report of a mutation in the mtDNA in bilateral striatal degeneration. It is possible that other cases of bilateral striatal degeneration have been caused by mutations in the mtATPase 6 gene or genes encoding other subunits of the mtATPase; and therefore the mtATPase genes should be examined in children with this condition.