Differences in educational performance and behaviour at age 10 years between 2900 hospitalized and 11,000 non-hospitalized children in the 1970 British Births Cohort were described previously. In the present analysis associations were found between the length of time children spent in hospital before the age of 5 years and performance on vocabulary tests at age 5 and age 10. Children whose first admission occurred between 2 and 5 years of age were particularly likely to show this association. There was also a relationship between the length of preschool hospitalization and reading and mathematics attainment at age 10. It was the number of times a child was admitted to hospital before the age of 5, rather than the length of stay, which was associated with antisocial and anxious behaviour at age 5. The association was evident when the first admission occurred between 2 and 5 years of age and not before age 2. There was no association with such behaviour at age 10 once social and family factors and readmission between ages 5 and 10 were taken into account. There was a strong association between admission between ages 5 and 10 and behaviour scores at age 10. The results are interpreted as providing evidence for a need to develop the hospital educational service to ameliorate these associations and to review schemes which reduce the anxiety-provoking aspects of hospitalization for young children.