Of 107 consecutive patients with genetically-determined retinitis pigmentosa, 23 were provisionally diagnosed as having inherited the disease in an X-linked fashion. 42 affected males and 61 females were examined, and from the data obtained the following conclusions were drawn: (1) X-linked retinitis pigmentosa exists and is distinct from choroideremia. (2) In contrast to the results of previous surveys, X-linked retinitis pigmentosa is a common form of this disease and over 20 per cent. of retinitis pigmentosa is probably transmitted in an X-linked manner. (3) (a) In contradistinction to the findings of previous investigators, most if not all adult heterozygous females have detectable degenerative changes in the ocular fundus. (b) The ocular changes in heterozygous females are most easily detected by fundus examination, visual field testing, dark adaptation measurements, and estimation of retinal rhodopsin concentration. The single most frequent abnormality is peripheral retinal pigment epithelial atrophy, which is found in all adult heterozygous females. (c) The pattern of retinal dysfunction in heterozygous females, and in particular preservation of the ocular electrical responses, suggests that the disease in women is qualitatively different from that in men and in other genetic forms of retinitis pigmentosa. There is some evidience that the disease in heterozygous women is patchy. (d) Degeneration in heterozygous females is usually symmetrical, but great variation was found in the severity of degeneration amongst heterozygotes of similar ages. No non-genetic influences were found to account for this. No evidence came to light by which the importance of X-chromosome inactivation could be assessed in determining the phenotype of heterozygous women. (4) No evidience is available to determine the number of X-linked genes transmitting the disease.