The maternal inheritance in Leber optic atrophy suggests that it may be caused by a cytoplasmic or mitochondrial defect. However, the strong male bias and the strict tissue specificity can not be readily explained by a single mitochondrial gene defect alone. Wallace suggested a hypothesis that the disease could be the result of an interaction between an X-linked gene and a mitochondrial DNA defect. Linkage relationships between Leber optic atrophy and 15 X-chromosome markers were analyzed in three large Tasmanian families. The results of two-point linkage analysis showed no close linkage between Leber optic atrophy and any of the 15 markers. The results of multipoint linkage analysis suggested the exclusion of the assumed X-linked gene from almost the whole X chromosome in these families.