This study examined inter-rater reliability of the Functional Mobility Scale (FMS) for children with cerebral palsy (CP) and the presence of rater bias. A consecutive sample of 118 children with CP, 2-18 years old (mean 10.3 years, SD 3.6), was recruited from a hospital setting. Children were classified using the gross motor function classification system (GMFCS) with 13 in Level I, 49 in Level II, 44 in Level III, and 12 in Level IV. Each child was independently scored on the FMS by two raters. Raters were randomly assigned from a sample of 44 orthopaedic surgeons, hospital-based physiotherapists, and community-based physiotherapists. Quadratic weighted kappa coefficients for mobility ratings varied from 0.86 to 0.92 for the three distances, indicating substantial chance corrected agreement. Levels of agreement were similar when administering the scale in person and by telephone, suggesting that the FMS can be administered by either method. There was a tendency for surgeons to rate mobility higher than physiotherapists, however, only one of the comparisons was statistically significant. The FMS is a reliable tool that can be used by clinicians to assess mobility in children with CP.