This study aimed to determine the probability that a child with cerebral palsy (CP) will lose or gain ambulatory ability through adolescence and young adulthood. We analyzed retrospectively data from 1987 to 2002 on Californians with CP initially aged 10 years (SD 0.9y; n=7550 [4304 males, 3246 females]) and 25 years (SD 0.8y; n=5721 [3261 males, 2460 females]) who had varying levels of ambulatory ability (initial Gross Motor Function Classification System Levels I-IV). We used the Aalen-Johansen estimator to estimate probabilities of transition to other levels of ambulatory ability in the future. Those who walked and climbed stairs without difficulty at age 10 had only a 23% chance of decline (to requiring a handrail to manage stairs, or worse) 15 years later. Those who ambulated with some difficulty but did not use a wheelchair had a significant chance (33%) of improvement (to being able to walk unsteadily alone at least 3m or better) and only a small chance (11%) of becoming non-ambulatory. Those who used a wheelchair were more likely to lose ambulatory ability (34%) or die (6%). Those who walked and climbed stairs well at age 25 were likely to maintain that ability 15 years later (76%), while those needing support to climb stairs were more likely to lose ability. Improvement in ambulation after age 25 was unlikely. Children and young adults with CP are likely to maintain their ambulatory ability during their next 15 years. Some who ambulate with difficulty at age 10 may improve through adolescence, but those who use a wheelchair are more likely to decline. By age 25 improvement in ambulation is unlikely and decline more likely. Most, however, will not change over the next 15 years.