Purpose. The purpose of this study was to examine the effect of progressive resistance training on leg strength, aerobic capacity and physical function in persons with Down syndrome (DS). Method. Thirty persons with DS (age 28 SD 8 years) were assigned to an intervention or control group. The intervention group performed resistance training 2 days per week for 10 weeks. Participants performed tests to measure isometric and isokinetic knee extensor and flexor peak torque, peak aerobic capacity and timed performance on chair rise, walking and stair ascent and descent. Result. Persons with DS receiving the intervention significantly increased their isokinetic knee extensor and flexor peak torque [Absolute change (post minus pre-value) was 17.0 SD 29.6 and 12.6 SD 18.9 N m, respectively] and isometric knee extensor peak torque at angles of 45 degrees (2.9 SD 23.2 N m), 60 degrees (3.0 SD 22.9 N m) and 75 degrees (14.2 SD 30.0 N m). These changes were significantly greater than in the control group. In addition, the time to ascend (-0.3 SD 0.8 s) and descend (-0.6 SD 0.9 s) stairs significantly decreased in the intervention group compared to the control group. Conclusion. These findings show that progressive resistance training is an effective intervention for persons with DS to improve leg strength and stair-climbing ability.