Background: Leber congenital amaurosis (LCA) is an infrequently encountered congenital form of retinitis pigmentosa with marked genetic and clinical heterogeneity. Thus far, 10 genes have been identified in this disorder since 1996. In the future, LCA may become treatable by gene and/or pharmacological intervention, and these therapies will likely be gene specific, giving major significance to rapid gene identification and genephenotype studies. Objective: To test the hypothesis that parents of patients with LCA have identifiable electroretinographic and psychophysical changes. Subjects, Materials, and Methods: Complete eye examinations and electroretinographic studies were performed on 2 sets of parents whose offspring were diagnosed as having LCA and who were found to carry a mutation in 1 of the 10 LCA genes - GUCY2D. One set of parents also underwent static perimetry threshold measurements. Results: We found that single flash-light-adapted a- and b-wave amplitudes, 30-Hz flicker, or both cone signals were significantly decreased in amplitude in 4 heterozygotes, while 2 parents showed delayed 30-Hz flicker implicit times. Electroretinographic rod-mediated signals were normal in 2 of the heterozygotes, but subnormal in 2. Static perimetry testing showed normal thresholds in the 2 heterozygotes tested. Main Outcome Measures: Single flash-light-adapted a- and b- wave amplitudes and implicit times, 30- or 32-Hz flicker amplitudes and implicit times, rod-mediated signals, and dark-adapted, rod-mediated thresholds. Conclusions: Some carrier parents of patients with LCA and a GUCY2D mutation develop measurable, cone and possibly rod abnormalities most consistent with a mild conerod dysfunction. This correlates well with the known retinal expression pattern of GUCY2D, which is considerably higher in cone compared with rod photoreceptor cells.