Background: Children with cerebral palsy (CP) are at greater risk for a limited intellectual development than typically developing children. Little information is available which children with CP are most at risk. This study aimed to describe the development of non-verbal intellectual capacity of school-age children with CP and to examine the association between the development of non-verbal intellectual capacity and the severity of CP. Methods: A longitudinal analysis in a cohort study was performed with a clinic-based sample of children with CP. Forty-two children were assessed at 5, 6 and 7 years of age, and 49 children were assessed at 7, 8 and 9 years of age. Non-verbal intellectual capacity was assessed by Raven's Coloured Progressive Matrices (RCPM). Severity of CP was classified by the Gross Motor Function Classification System, type of motor impairment and limb distribution. MANOVA for repeated measurements was used to analyse time effects and time x group effects on both RCPM raw scores and RCPM intelligence quotient scores. Results: The development of non-verbal intellectual capacity was characterised by a statistically significant increase in RCPM raw scores but no significant change in RCPM intelligence quotient scores. The development of RCPM raw scores was significantly associated with the severity of CP. Children with higher levels of gross motor functioning and children with spastic CP showed greater increase in raw scores than children with lower levels of gross motor functioning and children with dyskinetic CP. Conclusions: Children with CP aged between 5 and 9 years show different developmental trajectories for non-verbal intellectual capacity, which are associated with the severity of CP. The development of non-verbal intellectual capacity in children with less severe CP seems to resemble that of typically developing children, while children with more severe CP show a limited intellectual development compared to typically developing children.