The cellular basis of the hypersensitivity of the S (blue) cone system in patients with enhanced S cone syndrome was examined by analyzing ERGs from three patients. The patients had large alpha-waves in response to the blue and white flashes. These alpha-waves were shown to be driven nearly entirely by the S cones. Although these S cone alpha-waves were 4-6 times the size of the normal L/M cone alpha-wave, they are of the same form, and could be quantitatively described with the same model previously shown to fit cone alpha-waves. We propose that the retina of these patients has many more S cones than the normal retina and that these cones replace some of the normal L/M cones and many of the rods.