We detected mild afferent pupillary defects with the "swinging flashlight" test in 4 out of 45 amblyopic patients. Our study was designed to minimize the effect of observer bias and to control for the difficulty of testing the pupils in young children. We found afferent defects in both strabismic and anisometropic amblyopes. There was no apparent relationship between pupillary response and visual acuity: afferent defects were noted in association with better than 20/100 vision in three cases but were absent in a majority of patients with profound visual loss. Vision was improved by occlusion therapy in two amblyopes with pupillary abnormalities. We regard the occurrence of afferent pupillary defects as evidence for a physiological disturbance at the retinal level in at least some cases of amblyopia. Our findings suggest that the extent of retinal involvement in amblyopic eyes is independent of reduction in acuity, to which primary cortical abnormalities may contribute as well.