The quality of information collected during certification for blindness and partial sight was assessed using a combination of review of the literature on registration and analysis of 17,695 BD8 certificates received by the Office of Population Censuses and Surveys (OPCS) between June and November 1990. Three problems were identified which affect the epidemiological function of these data: interpretation of the definitions of blindness and partial sight has changed over time; the degree of under-certification is unknown (although it could be as high as 64% for blind and 77% for partially sighted people); and the cause of visual disability recorded by the ophthalmologist may not always be adequately transmitted into the statistical analyses. Guidelines for completion of part 5 of the BD8 form need to be developed. Strategies for improving the quality of these data are discussed.