To determine whether congenital tritanopia and dominantly inherited juvenile optic atrophy (DIJOA) are the same clinical entity, we used electroretinograms of the blue-sensitive cone system (blue cone ERGs), comparing those of two patients with congenital tritanopia from two pedigrees with those of four patients with DIJOA from two pedigrees. The examinations also included visual acuity and visual field tests, fundus examination, the dark-adaptation test, and several color vision tests. The blue cone ERG confirmed a difference between the two groups; it was unrecordable in the patients with congenital tritanopia but within the normal range in those with DIJOA. We believe that congenital tritanopia and DIJOA are distinct disease entities and that the blue cone ERG is a key factor in the differential diagnosis.