AIM: The aim of this study was to investigate whether individualized resistance training improves the physical mobility of young people with cerebral palsy (CP). METHOD: Forty-eight participants with spastic diplegic CP (26 males, 22 females; mean age 18y 1mo, SD 1y 11mo) classified as level II or III on the Gross Motor Function Classification System were allocated randomly to progressive resistance training or usual-care control. Resistance training was completed twice weekly for 12 weeks at a community gymnasium under the supervision of a physiotherapist. Exercises were based on instrumented gait analysis and targeted muscles contributing to walking difficulties. Outcomes at 12 weeks and 24 weeks included objective measures of mobility (6-min walk test, instrumented gait analysis, and Gross Motor Function Measure dimensions D and E), participant-rated measures of mobility (Functional Mobility Scale and Functional Assessment Questionnaire), and muscle performance. RESULTS: The strength of targeted muscles increased by 27% (95% CI 8-46%) compared with control group. There were no between-group differences in any objective measure of mobility at 12 weeks (6-min walk test: mean difference 0.1m; 95% CI -21 to 21m) or at 24 weeks. Participant-rated mobility improved (Functional Mobility Scale at 5m: mean 0.6 units; 95% CI 0.1-1.1 units; Functional Assessment Questionnaire: 0.8 units; 95% CI 0.1-1.6 units) compared with control group at 12 weeks. INTERPRETATION: Individualized progressive resistance training increased strength in adolescents and young adults with spastic diplegic CP. Despite participant-rated benefits, the increased strength did not result in objective improvements in mobility.