A new color vision test distinguishes patients with X-chromosome-linked blue cone monochromatism from those with autosomal recessive rod monochromatism. The test consists of two instructional and four test plates. Each test plate has three identical blue-green arrows and one purple-blue arrow; test plates differ from one another only with respect to the chroma of the purple-blue arrow. All five patients with blue cone monochromatism, aged 5 to 31 years, easily distinguished the purple-blue arrow on all four test plates, whereas none of the seven patients with rod monochromatism, aged 6 to 60 years, could distinguish the purple-blue arrow on all four plates. If a boy has a reduced visual acuity, normal rod electroretinograms, and 30-Hz cone electroretinograms reduced more than 97% below normal, this test can be used to determine whether his condition is an X-chromosome-linked one or an autosomal recessive one.