Using a guessing paradigm we measured visual sensitivity in the blind and normal half-fields of four cerebrally hemidecorticated patients. In the blind field, sensitivity was reduced by approximately 3 long units. Stimuli which produced significant detection also evoked conscious sensations of light and colour. Control experiments showed that although sensitivity in the blind field depended in a normal fashion on background luminance it was independent of the luminance of a local platform, and showed no spatial summation. This residual vision can be explained by intraocular light diffusion and reflection.