PURPOSE: Age-related macular degeneration (AMD) is the most commonly occurring cause of visual loss in people registered as blind or partially sighted. There are no nationally representative data on the prevalence of AMD in the British population. We aimed to estimate the prevalence of AMD causing visual impairment in people 75 years or older in Britain. DESIGN: Population-based cross-sectional study. PARTICIPANTS: Thirteen thousand nine hundred people 75 years or older in 49 practices taking part in the Medical Research Council Trial of the Assessment and Management of Older People in the Community. METHODS: Trial nurses tested visual acuity in everyone 75 years or older in participating practices. We collected data on the cause of visual loss for everyone who was visually impaired. We obtained these data from review of the general practice medical notes and by sending a questionnaire to the hospital ophthalmologist. Visual impairment was defined as a binocular acuity of less than 6/18. MAIN OUTCOME MEASURE: Prevalence of AMD causing visual impairment. RESULTS: There were 976 visually impaired people for whom a cause of visual loss was established. Of these, 516 (53%) had AMD as a cause of visual loss. We estimate that 3.7% (95% confidence interval, 3.2%-4.2%) of the population 75 years or older and 14.4% (11.6%-17.2%) of the population 90 years or older are visually impaired due to AMD. There are an estimated 192 000 people 75 years or older visually impaired due to AMD in the United Kingdom (95% confidence interval, 144 000-239 000). CONCLUSION: Our results, from the largest and most representative study of the causes of vision loss in older people in the British population, confirm the substantial burden of AMD in people 75 years and older. As the population ages, this problem will get worse. The needs of this group for vision aids and other support in the community should be addressed; research on the causes of AMD and possible preventive measures should be given priority.