Blue cone monochromatism (BCM) is a rare X-linked recessive disease characterized by an absent function of L and M cones and a normal function of S cones and rods. The estimated prevalence is 1 in 100,000 individuals, and males are predominantly affected. The patients usually present at birth or in early infancy. The clinical features include impaired color vision (patients only see colors in the blue range of light) and poor visual acuity (between 20/80 and 20/200), photophobia, pendular nystagmus (which may improve over time), and myopia. The condition is typically stationary, but progressive central retinal atrophy may be seen later in life. The presentation is similar to rod monochromatism (achromatopsia), an autosomal recessive disease that affects all three types of cones. However, BCM patients have better visual acuity, preserved tritan discrimination, and myopia (as opposed to hyperopia in achromatopsia). Electrophysiology, psychophysical testing (using Berson plates), and family history (x-linked recessive vs. autosomal recessive) help distinguish the two conditions.