This research aimed to investigate if age of onset affects the pathogenesis of amblyopia. Amblyopic subjects were compared with normal controls. Strabismic amblyopes were assigned to early or late onset groups on the basis of detailed clinical history. Confirmatory histories were obtained from parents where possible. Contrast Sensitivity (CS) to a 3.2 cyc/deg (cpd) sinusoidal grating pattern was recorded. Monocular pattern appearance visual evoked potentials (VEP) at five contrast levels were recorded. Luminance and chromatic CS, motion onset and colour VEPs were recorded. Late onset amblyopes showed reduced CS at 3.2 cpd for the amblyopic, but increased CS for the fellow eye compared to normal. Late onset amblyopic eye VEP CII latencies were longer and amplitudes smaller than normal. CII responses in amblyopic and fellow eyes of the early onset group were of shorter latency and smaller amplitude than normal. Late onset amblyopes had reduced luminance CS of the amblyopic compared to fellow eyes. Fellow eye chromatic CS was lower than normal in all eyes. The (M-P)/M ratio was greater in the late onset amblyopic eyes. This ratio was increased in all fellow eyes compared to normal. All amblyopes had shorter N200 latencies for both eyes compared to normal. Late onset amblyopic eye VEP N130 latency was longer than in early amblyopes. Early onset amblyopic eye N130 amplitude was smaller than fellow eye amplitude. Results were independent of visual acuity. Different patterns of contrast abnormality occur in early and late onset amblyopes. Late onset amblyopes have a greater parvocellular (P) deficit. Magnocellular (M) results suggest an enhancement of this pathway in all amblyopes. Fellow eyes of amblyopes are abnormal. Examining children during treatment for amblyopia would develop these hypotheses. The changes observed in the M and P pathways could be used to develop treatment strategies in the future.