The result of an epidemiological investigation, carried out under the sponsorship of the Commission for the European Communities (CEC) and its Committee on Medical and Public Health Research (CRM) in 9 countries [United Kingdom, Denmark, Belgium, West Germany, Italy, the Netherlands, Ireland, Luxembourg and France] in order to determine the prevalence of childhood deafness. The design of the study specified that all children born during the year 1969 would be included if their hearing loss averaged 50 dB or worse in the better ear. Most of the children were enumerated in 1977 when 8 yr old. The prevalence of the degree of deafness in the CEC was 0.9/1000 live births in 1969. There were more deaf children born in the winter than in the summer months. Perceptive hearing loss accounts for 92% of all the cases of deafness, and Rubella was the largest single cause of the deafness where etiology was known. If perceptive deafness is considered as a whole, in 42% of the children no cause was reported. Of the children, 29% were reported as having associated handicaps. In 33% there was a mean hearing loss of 100 db or more. Of the children .apprx. 2/3 were suspected of being deaf by the time they reached their 3rd birthdays, and almost 50% had their deafness confirmed by the same age. For 33% there was a delay of up to 12 mo. between confirmation of their hearing losses and the issue of hearing aids. There were major differences in hearing and arrangements adopted by the different countries and in aid usage both in and out of school. There was no evidence of hearing in 10% of the children even when using hearing aids. Of the children .apprx. 2/3 attend special schools for the deaf and the remainder attend ordinary schools; there are marked differences between countries in the types of schools attended. Speech that was at least intelligible to strangers was reported in 47% of the children, and 32% could read like normal 8-yr-olds. There is a possible relationship between hearing aid usage out of school and intelligibility of speech.