A prevalence study of ataxia telangiectasia was conducted in the West Midlands, with a population of over 5 million. The prevalence in those aged 50 or less was found to be 1 in 514,000 and the birth frequency to be about 1 in 300,000. A genetic study of 47 families ascertained throughout the United Kingdom was carried out concurrently. A low parental consanguinity rate was found, no parents being first cousins or more closely related, whereas 10% had been expected. The incidence of ataxia telangiectasia in the 79 sibs of index cases was 1 in 7. These two features demonstrate that ataxia telangiectasia may not always be an autosomal recessive condition. Other possible explanations are that some cases are double heterozygotes or new dominant mutations.