The prevalence of Down syndrome by age and sex was established from questionnaires sent to heads of 30 registers in the United Kingdom covering a general population of over 7 million. Possible underascertainment and factors affecting prevalence were discussed. The overall prevalence of Down syndrome in the United Kingdom was 6.7 per 10,000 general population, representing approximately 30,000 affected individuals. Prevalence was also presented in 5-year age bands. We used two methods of projection and found no significant difference in their results. There was no indication of a sizable reduction in the future Down syndrome population. An incidence- and mortality-based projection method provided evidence that recent reductions in prevalence among the youngest age bands may be explained by changes in the maternal population.