We looked at the incidence and demography of nonaccidental head injury in children in a prospective population-based study of paediatric units in Scotland during 1998-99. Shaken impact syndrome occurs with an annual incidence of 24.6 per 100000 children younger than 1 year (95% CI 14.9-38.5). Cases are more common in urban regions, and during autumn and winter months. The risk of a child suffering non-accidental head injury by age 1 year is one in 4065. These brain injuries occur almost exclusively in young infants (median age 2.2 months).