Blue cone monochromatism (BCM) is a subtype of achromatopsia in which the blue cone mechanism predominates. Each of the four patients in this study had BCM proven by their having peak spectral sensitivities in the blue region of the visible spectrum (near 440 nm). Clinically, the diagnosis was suspected because of x-linked inheritance, the presence of acuities better than 20/200 in two patients and myopia ranging from -1.75 to -15.00 diopters in three patients. Congenital nystagmus was the presenting sign in three of the four patients. Examination of the fundi was uniformly normal. The distinctive spectral properties of BCM were demonstrated by the American Optical H-R-R and the Panel D-15 tests. All affected patients correctly identified three of the four blue-yellow plates and a variable number of the red-green plates in the American Optical H-R-R test. The study patients consistently made errors oriented along the protan and deutan axes but they made none along the tritan axis. The authors conclude that the results of these two color discrimination tests are useful in diagnosing BCM.