The aim of this study was to assess the stability of the Gross Motor Function Classification System (GMFCS) by examining whether children with cerebral palsy (CP) remain in the same level over time. Participants were 610 children with CP (342 males, 268 females; mean age 6y 9mo [SD 2y 10mo]), range 16mo–13y). Children were assessed 2 to 7 times (mean 4.3) at 6-month (children <6y old) or 12-month(children [ges ]6y old) intervals. Seventy-three per cent of children remained in the same level for all ratings. The weighted kappa coefficient between the first and last ratings was 0.84 for children less than 6 years old and 0.89 for children at least 6 years old, indicating excellent chance-corrected agreement. Children initially classified in Levels I and V were least likely to be reclassified. There was a tendency for children younger than 6 years who were reclassified to be done so to a lower level of ability. The results provide evidence of stability of the GMFCS.