Relative afferent pupillary defects (RAPD) were detected in 32.3% of patients with amblyopia by a modification of the swinging flashlight test and the synoptophore. After consideration of various clinical investigations the significant factors identified in patients showing a RAPD were: anisometropia, early age of onset where strabismus was present, level of visual acuity following treatment, longer period of occlusion therapy. These points bear similarities to the results of pattern electroretinograms (PERG) in amblyopes, and the possibility of the causative defect being at ganglion cell level is discussed. The effect of occlusion treatment cannot be predicted from the presence or absence of a RAPD.