We asked whether suppression in amblyopia could be accounted for by dichoptic masking as described in normals, operating in the presence of a contrast threshold difference between the two eyes. A dichoptic masking paradigm was employed to investigate binocular interaction in a mixed group of amblyopic subjects. Normal dichoptic masking was not seen after threshold differences between the two eyes were accounted for in the majority of subjects studied. We found that the binocular dysfunction did not merely follow as a consequence of the known monocular loss and that it depended upon the aetiology of the amblyopia and the spatial frequency of the stimulus.