QUESTION:Does progressive resistance training improve muscle strength and physical function in adolescents with Down syndrome? DESIGN:Randomised controlled trial with concealed allocation, assessor blinding, and intention-to-treat analysis. PARTICIPANTS:23 adolescents with Down syndrome (17 boys, 6 girls; mean age 15.6 +/- 1.6 years) were randomly assigned to either an experimental group (n = 11) or a control group (n = 12). INTERVENTION:The intervention was a student-led progressive resistance training program, comprising 6 exercises using weight machines performed twice a week for 10 weeks. Participants completed 3 sets of 12 repetitions of each exercise or until they reached fatigue. The intervention took place in a community gymnasium. The control group continued with their usual activities. OUTCOME MEASURES:The outcomes measured at baseline and immediately after the intervention phase were muscle strength (1 repetition maximum), a timed stairs test, and the grocery shelving task. RESULTS:The experimental group attended 90% of their scheduled sessions. They demonstrated improvement in lower limb muscle strength compared to the control group (MD 36 kg, 95% CI 15 to 58). There were no significant differences between the groups for upper limb muscle strength or physical function measures. No major adverse events were recorded. CONCLUSION:Progressive resistance training is a feasible and safe exercise option that can improve lower limb muscle strength in adolescents with Down syndrome. TRIAL REGISTRATION:ACTRN12608000261314.