It is well known that men and women may experience, perceptually and cognitively, the appearance of color differently. One of the possible physiological factors underlying these differences is a sexual dimorphism in the gene that encodes the photopigment of the long-wavelength-sensitive cones in the retina, manifest in a different frequency of expression in men and women. The p resent work describes a psychophysical experiment that revealed significant diffrences in color perception between men and women, and that consequently advises the separate treatment of the two populations. (c) 2007 Wiley Periodicals, Inc.