All developmental screening in Somerset is performed by general practitioners and health visitors. A retrospective review of a cohort of 1504 7 year old children living in semirural Somerset found that the development assessment by a health visitor at age 3 1/2 years had a sensitivity of 45% for identifying the 103 children with special educational needs, whereas the sensitivity of the preschool examination by a general practitioner was 56%. There was no relation between results of preschool developmental assessment and later reading ability. Of the 23 children in special schools, 22 had been identified independently of the developmental screening programme before starting school. Intervention was started at a mean age of 1 year 5 months (range 2 months to 3 years) for children with severe learning difficulties, and 3 years 9 months (2 years 3 months to 6 years 6 months) for children with moderate learning difficulties. The preschool medical examination revealed fairly minor medical problems: 29 of 81 children referred for specialist opinions were shown to be medically normal, and for only seven of the medically abnormal children was information about their conditions given to their teachers. In Somerset screening the development of all children at predetermined ages has not been very useful.