This study was designed to compare assessment with the functional outcome measures Gross Motor Function Measure (GMFM) and Pediatric Evaluation of Disability Inventory (PEDI) over time, in children with cerebral palsy (CP) undergoing selective dorsal rhizotomy combined with individualised physiotherapeutic interventions. Using the Gross Motor Function Classification System (GMFCS), 18 children with spastic diplegia were divided into two groups according to age-related severity of motor function impairment. Data were collected preoperatively, and at 6 and 12 months postoperatively. Both instruments were sensitive to changes in function over time in the series as a whole and in the group with milder impairment, although the PEDI detected significant changes earlier. In the group with more severe impairment, changes in function were detected only with the PEDI, not with the GMFM. Thus, the instruments are to be considered complementary tests, because they measure different aspects of function.