Using a simple assessment of language and a behaviour screening questionnaire, a one-in-four random sample of a total population of three-year-olds was screened for the presence of language delay and behaviour problems. A full psycho-developmental assessment was carried out in children with suspected language delay or a behaviour problem, and in a group of control children, and the sensitivity and specificity of the language screening-test was examined. The results showed the importance of investigating language delay in relation to nonverbal abilities. Of those children with a language age less than two-thirds of their chronological age, 37 per cent also had similarly delayed non-verbal mental abilities. 50 per cent of the children with a language age of less than 30 months were generally retarded in their non-verbal abilities. The estimated prevalence of delayed language development is 31 per 1000, of specific language delay 5-7per 1000, and of severe retardation 4-2 per 1000. Boys were most likely than girls to have language and general developmental delay.