In two patients with total acquired cortical colour blindness and in six control subjects we studied the binocular pupillary response to a variety of sharply defined coloured and grey displays that either had the same mean luminance as the background (isoluminant) or were of greater mean luminance. Despite their complete inability to identify or to discriminate between colours the patients, like the control subjects, showed a pupillary response to the structured coloured displays, even when they were masked by dynamic luminance changes. However, and unlike the control subjects, the patients showed no pupillary response when the coloured displays lacked sharp chromatic borders, as in Gabors or Gaussians. The results indicate that although chromatic processing still occurs in cortical colour blindness its function is solely to give rise to the detection of sharp boundaries which, in their case, can provide the perception of shape but not hue. In accordance with this, the patients could no longer describe the isoluminant borderless figures, which were often totally invisible to them despite their strong chromatic contrast with the background.