Choroideremia is a bilateral and progressive chorioretinal degenerative disease of X-linked inheritance. In Taiwan, the disease has rarely been reported. We recently found 8 male patients and 5 female carriers in two Chinese families with choroideremia and studied the clinical, psychophysical and electrophysiological properties of the disease. All patients revealed characteristic fundus and fluorescein angiographic changes in retinal pigment epithelium, and choroidal atrophy of variable extent. The severity of the functional impairments paralleled the fundus changes and the patients' age. However dark adaptation and the visual field were affected earlier than visual acuity and color sensation. In addition, rod cell impairment was found to be more severe than cone cell impairment in the electroretinographic studies. On the other hand, all carriers revealed similar diffuse punctate pigmentary changes and only mild abnormalities in dark adaptation. These findings may suggest that in choroideremia patients the photoreceptor cells are very sensitive to the influence of the choroidal atrophy, and that rod cells may be even more vulnerable than cone cells.