AIMS: To examine the relative roles of genetic and environmental factors in central retinal thickness, by performing a classical twin study. METHODS: 310 subjects were recruited from the TwinsUK adult registry at St Thomas' Hospital. Optical coherence tomography (Zeiss, stratus OCT3) was used to measure the average retinal thickness in the central 1 mm diameter area. The covariance of central retinal thickness (CRT), within MZ and DZ twin pairs, was compared and genetic modelling techniques were used to determine the relative contributions of genes and environment to the variation in CRT observed in this population. MAIN OUTCOME MEASURE: CRT (average retinal thickness in the central 1 mm diameter area, centred on the fovea). RESULTS: The mean CRT of all subjects was 212.1 microm (range 165-277). CRT was statistically related to refractive error, with increasing myopia associated with a thinner CRT. CRT was more highly correlated within MZ twin pairs (r = 0.88) than with DZ twin pairs (r = 0.58), suggesting a genetic role. A model combining additive genetic and unique environmental factors provided the best fitting model and gave a heritability estimate of 0.90. CONCLUSION: Genetic factors appear to play an important role in CRT, with a heritability estimate of 0.90.