This study explored the relationships between the Gross Motor Function Classification System (GMFCS), limb distribution, and type of motor impairment. Data used were collected in the Ontario Motor Growth study, a longitudinal cohort study with a population-based sample of children with cerebral palsy (CP) in Canada (n=657; age 1 to 13 years at study onset). The majority (87.8%) of children with hemiplegia were classified as level I. Children with a bilateral syndrome were represented in all GMFCS levels, with most in levels III, IV, and V. Classifications by GMFCS and 'limb distribution' or by GMFCS and 'type of motor impairment' were statistically significantly associated (Pearson's chi2 p<0.001), though the correlation for limb distribution (two categories) by GMFCS was low (tau-b=0.43). An analysis of function (GMFCS) by impairment (limb distribution) indicates that the latter clinical characteristic does not add prognostic value over GMFCS. Although classification of CP by impairment level is useful for clinical and epidemiological purposes, the value of these subgroups as an indicator of mobility is limited in comparison with the classification of severity with the GMFCS.