The present study investigated the reliability of self-reported rating of Gross Motor Function Classification System (GMFCS) levels compared with professional rating, and changes in gross motor function over time, in adults with cerebral palsy. Twenty-nine females and 33 males aged between 18 years 5 months and 62 years 11 months (mean age 34y 7mo [SD 10y 6mo]) participated in the study. Participants rated their current gross motor function using the GMFCS and reported their judgement of their gross motor function at age 10 to 12 years. The project leader, a physical therapist, also classified participants' current GMFCS levels and conducted a chart review on all accessible medical records of participants' gross motor function when they were 10 to 12 years old, rating the GMFCS level accordingly. Intraclass correlation coefficients (ICC) between self-reported and professional ratings showed excellent agreement (ICC=0.93–0.95, 95% confidence interval [CI] 0.89–0.97). More than half the participants experienced a stable gross motor function from the age of 10 to 12 years to the present. Those at GMFCS Levels II and III at the age of 10 to 12 years (according to the professional rating) had significant change for the worse in gross motor function over time, with odds ratios of 9.30 (95% CI 1.2–73.0, p=0.03) and 7.00 (95% CI 1.1–43.0, p=0.04) respectively. Interview data on circumstances regarding changes in gross motor function since childhood are also reported. Changes in GMFCS level were mostly associated with physical or social environmental factors.