Three patients with a bull's-eye macular lesion and other signs characteristic of cone dystrophy gave an unusual ERG finding. In response to a white flash of moderate intensity the scotopic b-wave amplitude was considerably larger than normal. One patient had elevated rod thresholds and nyctalopia, while the other 2 had normal rod sensitivity associated with the supernormal scotopic b-wave amplitude. In the latter 2 patients the abnormal ERG pattern was unchanged for 4 years and 7 years respectively. This atypical finding, of a supernormal scotopic b-wave amplitude in response to light of moderate intensity, appears to characterise a subgroup of patients with cone dystrophy, probably of autosomal recessive inheritance. The pathogenesis of the abnormal ERG remains uncertain.