A community-based investigation of the nature, characteristics and evolution of speech and language delay in a sample of 3-year-olds is being carried out in Cambridgeshire. 1936 parents completed a preschool language checklist (PLC) to identify children at risk of language difficulties. Two hundred and seventy-seven children at risk together with 148 controls completed a series of preliminary face-to-face standard language tests to determine expressive, receptive and phonological skills. Concordance between the Cambridge Language and Speech Project (CLASP) identification and speech therapy involvement suggests that the overall (CLASP) screening procedure identified a number of children that current surveillance had missed and support the conclusion that the PLC may be a useful adjunct for child health care services as an aid in prioritising children for referral to speech therapy services. Children with scores of 1 to 3 at 36 months should be reviewed at 39 months, and those with 4 or more should be a high priority for referral. Preliminary examination of the impairment profile suggests that children with language impairments rather than pure speech impairments at 36 and 39 months are more likely to have a broader range of overall language-related deficit.