X-linked incomplete achromatopsia (XIA), also called blue-cone monochromacy (BCM), is a rare cone disorder that most commonly results either from one of two conditions.. The first condition is a deletion of the locus control region (LCR) which is a critical DNA element that lies upstream of the L and M photopigment gene array on the X-chromosome and is necessary for expression of the photopigment genes. The second condition is an inactivating point mutation within the coding sequence of the remaining photopigment gene in an array from which all but one gene has been deleted. Many previous studies have concluded that affected individuals either have only rods and S-cones (Blackwell & Blackwell, 1957, 1961; Daw & Enoch, 1973; Hess et al., 1989) or have rods, S-cones, and another cone type that contains the rod pigment (Pokorny et al., 1970; Alpern et al., 1971). However, Smith et al. (1983) described individuals with XIA who had residual L-cone function. Here we report results for a subject with XIA who appears to have residual M-cone function. Genetic analysis revealed that he had apparently normal genes for M-cone photopigment thus leaving open the possibility that he has a contribution to vision based on expression of these genes at a very low level.