A register of retinoblastoma is being compiled with the aim of complete ascertainment of all cases with a recorded diagnosis of retinoblastoma in the United Kingdom. Sources of ascertainment include patients seen at St Bartholomew's and Moorfields Eye Hospitals, cases recorded in regional cancer registries, and pathology reports. Ascertainment of the Hospital series is complete, but that of the rest of the country is still incomplete. Comparison between the proportions of bilateral and unilateral cases in an age cohort in the Hospital series and the country as a whole shows that the Hospital series is biased towards an excess of bilateral cases.