Aims: To determine if asymptomatic carriers from a previously identified large pedigree of the Leber’s hereditary optic neuropathy (LHON) 11778 mtDNA mutation have colour vision deficits. Methods: As part of a comprehensive analysis of over 2CXD members of a large Brazilian LHON pedigree spanning seven generations, colour vision tests were obtained from 91 members. Colour vision was tested one eye at a time using the Farnsworth-Munsell 100 (FM-100) hue colour vision test. The test was administered under uniform conditions, taking into account: ambient light levels, daylight colour temperature of 6700 kelvin, and neutral uniform background. Tests were scored using the FM-100 MS-Excel computer scoring program. Defects were determined and categorised as tritan, deutan, or protan. Categorisation of each dyschromatopsia was based on review of demonstrated axis computer generated plots and age adjusted error scores which coincided with Verriest 95%confidence intervals. Only the axis with the greatest magnitude error score was used to classify the defect. 55 of the 91 test subjects were LHON mtDNA 11778 J haplotype mutation carriers, proved by mtDNA analysis. The remaining 36 subjects were age matched non-blood relatives (off pedigree), who served as controls. Results: 27 of 55 carriers (49.10%) were shown to have colour vision defects in one or both eyes. 13 of the 27 (48%) abnormal tests in the carrier group were tritan defects and the remaining 14 (52%) were deutan defects. Nine of the 27 (33%) abnormals in the carrier group were identified as having bilateral defects. Six of these were deutan, and the remaining three were tritan dyschromatopsias. Only six of the 36 (16.66%) age matched controls were found to have any type of dyschromatopsia. Five (83.3%) of these were deutan defects. The remaining one was a tritan defect. The difference between the two groups using χ2 test with one degree of freedom was statistically significant with a p value less that 0.001. Conclusions: Until now, LHON has always been characterised by a sudden, devastating vision loss. Asymptomatic carriers, those without vision loss, were considered unaffected by the disease. It now appears that asymptomatic carriers of the LHON mutation are affected by colour vision defects and maymanifest other subtle, yet chronic, changes. Methods: As part of a comprehensive analysis of over 2CXD members of a large Brazilian LHON pedigree spanning seven generations, color vision tests were obtained from 91 members. Color vision was tested one eye at a time using the Farnsworth-Munsell 100 (FM-100) hue color vision test. The test was administered under uniform conditions, taking into account: ambient light Levels, daylight color temperature of 6700 kelvin, and neutral uniform background. Tests were scored using the FM-100 MS-Excel computer scoring program. Defects were determined and categorized as tritan, deutan, or protan. Categorization of each dyschromatopsia was based on review of demonstrated axis computer generated plots and age adjusted error scores which coincided with Verriest 95% confidence intervals. Only the axis with t he greatest magnitude error score was used to classify the defect. 55 of the 91 test subjects were LHON mtDNA 11778 J haplotype mutation carriers, proved by mtDNA analysis. The remaining 36 subjects were age matched non-blood relatives (off pedigree), who served 13 of the 27 (48%) abnormal tests in the carrier group were tritan defects and the remaining 14 (52%) as controls. Results: 27 of 55 carriers (49.10%) were shown to have color vision defects in one or both eyes. Nine of the 27 (33%) abnormals in the carrier group were identified as having bilateral defects. Six of the were deutan, and the remaining three were tritan dyschromatopsias. Only six of the 36 (16.66%) age matched Controls were found to have any type of dyschromatopsia. Five (83.3%) of these were deutan defects. The remaining one was a tritan defect. The difference between the two groups using χ2 test with one degree of freedom was necessarily significant with ap value less that 0.001. Concl usions: Until now, LHON has always been characterized by a sudden, devastating vision loss. Asymptomatic carriers, those without vision loss, were considered unaffected by the disease. It now appears that asymptomatic carriers of the LHON mutation are affected by color vision defects and maymanifest other subtle, yet chronic, changes.