This study compared the long-term outcome of single-event multilevel surgery in spastic diplegic cerebral palsy (CP) on the basis of sex. We hypothesized that boys would have a worse outcome than girls. Thirty-four children (19 boys and 15 girls) with diplegic spastic CP and a minimal follow-up of 10 years were included. The Gillette Gait Index was the main outcome measure. We found no differences in surgical treatment, and both groups initially benefited from the surgery. However, although girls maintained the enhanced level of walking, walking ability in boys deteriorated constantly. Such a finding suggests that sex might have an important influence on treatment outcomes in children with CP.