Single-event multilevel surgery is considered the standard of care to improve gait and functioning of children with spastic diplegic cerebral palsy. However, the evidence base is limited. This pilot study is the first randomized controlled trial of single-event multilevel surgery, to our knowledge.Nineteen children (twelve boys and seven girls with a mean age of nine years and eight months) with spastic diplegia were enrolled. Eleven children were randomized to the surgical group and eight, to the control group. The control group underwent a program of progressive resistance strength training. The randomized phase of the trial concluded at twelve months. The control group then exited the study and progressed to surgery, whereas the surgical group continued to be followed in a prospective cohort study. The primary outcome measures were the Gait Profile Score (GPS) and the Gillette Gait Index (GGI). Secondary outcome measures were gross motor function (Gross Motor Function Measure-66 [GMFM-66]), functional mobility (Functional Mobility Scale [FMS]), time spent in the upright position, and health-related quality of life (Child Health Questionnaire [CHQ]).A total of eighty-five surgical procedures were performed, with a mean of eight procedures per child (standard deviation, four). The surgical group had a 34% improvement in the GPS and a 57% improvement in the GGI at twelve months. The control group had a small nonsignificant deterioration in both indices. The between-group differences for the change in the GPS (-5.5; 95% confidence interval, -7.6 to -3.4) and the GGI (-218; 95% confidence interval, -299 to -136) were highly significant. The differences between the groups with regard to the secondary outcome measures were not significant at twelve months. At twenty-four months after surgery, there was a 4.9% increase in the GMFM-66 score and improvements in the FMS score, time spent in the upright position, and the physical functioning domain of the CHQ in the surgical group.This study provides Level-II evidence that single-event multilevel surgery improves the gait of children with spastic diplegic cerebral palsy twelve months after surgery. Improvements in other domains, including gross motor function and quality of life, were not observed until twenty-four months after surgery.