Leber's hereditary optic neuroretinopathy (LHON) was the first human disease for which mitochondrial inheritance was demonstrated. Analysis of genealogies, however, suggests the existence of an interacting X-linked factor, and linkage to DXS7 was recently described. We tested this location in four LHON families, with DXS7 and two flanking markers, OTC and DXS426. We found recombinations with DXS7 in two families and with DXS426 in one. The two point lod scores to DXS7 were negative with all the allele frequencies for the X-linked factor tested (q = 0.5; 0.35; 0.05).