A special conditions register (SCR) linked to the child health system's register of ail children has been in use in West Sussex since 1977. This paper describes the aims, organisation, and use of the SCR and gives examples of the aggegated data that may be obtained. Of the 155 000 children aged 0-17 resident in West Sussex in 1990, 4-3% were included on the SCR. Altogether 45.7% of children on the SCR had physical conditions with mild or no disability and 17-2% had moderate educational problems. The prevalence of severe hearing loss as defined was 1-7 per 1000 aged 5-17. The prevalence of diabetes mellitus was 1.2 per 1000 children aged 0-17. Validation of the SCR for diabetes mellitus found 35/36 of the eligible children were correctly registered and no child was incorrectly included. The conflicting priorities for maintaining a register for the care of individual children, for service planning, and for epidemiological research are discussed.