Three mutations in mitochondrial DNA complex I genes are responsible for over 90% of Leber hereditary optic neuropathy (LHON) cases in Europe. A family with two LHON mutations--practically homoplasmic 11778G>A and varying levels of 3460G>A--was found during analysis of Polish patients. DNA and visual acuity was analyzed in four affected brothers and their unaffected sister and mother as well as in their step brother. Four male patients experienced vision loss around the age of 20 while for their step brother the onset was late--at the age of 33. No additional neurological symptoms were observed and both women were completely asymptomatic. The mutation occurred in a haplogroup H background, the most common one in both the Polish population and among patients. Double LHON mutations are extremely rare, and this particular combination has not been previously described in the literature.