This randomized clinical trial evaluated the effects of a home-based, six-week strength-training programme on lower limb strength and physical activity of 21 young people (11 females, 10 males; mean age 13 years 1 month, SD 3 years 1 month; range 8 to 18 years) with spastic diplegic cerebral palsy (CP) with independent ambulation, with or without gait aids; (Gross Motor Function Classification System levels I to III). Compared with the 10 controls, the 11 participants in the strength-training programme increased their lower limb strength (combined ankle plantarflexor and knee extensor strength as measured by a hand-held dynamometer) at 6 weeks (F(1,19)=4.58, p=0.046) and at a follow-up 12 weeks later (F(1,18)=6.25, p=0.041). At 6 weeks, trends were also evident for improved scores in Gross Motor Function Measure dimensions D and E for standing, running and jumping, and faster stair climbing. A relatively short clinically feasible home-based training programme can lead to lasting changes in the strength of key lower-limb muscles that may impact on the daily function of young people with CP.