Aim: To determine the degree of stability in the Gross Motor Function Classification System (GMFCS) for children with cerebral pares (CP) and to analyze factors associated with changes of the over time, in Stockholm between the year 2000 and 2019. Method: A register study on 768 children with at least two GMFCS ratings, linear regression analysis was used to study factors associated to a change in GMFCS level. Results: 72% of the children kept the same GMFCS level. A change in GMFCS level was most common for children in GMFCS level II (68%). The first change in GMFCS level happened most commonly between the ages 2 and 4. Initial GMFCS level (Beta 0.127; p < 0.001) and one or more intensive training periods with a physiotherapist (Beta 0.097; p = 0.018) were associated with a change in GMFCS level. Interpretation: Most children with CP remain stable in their GMFCS level, but for those classified in level II, a change occurs for almost seven out of ten. Classifications made before the child turns four are less certain than those made later. Further studies are needed to clarify if occurrence of intensive training is the cause or result of the change in GMFCS level. © 2020 The Author(s). Published with license by Taylor & Francis Group, LLC.