Aim  To determine the effectiveness of an 8-week internet-based, lifestyle physical-activity intervention for adolescents with cerebral palsy (CP).

Method  A randomized controlled trial using concealed allocation with blinded assessments at baseline, 10, and 20 weeks. Forty-one adolescents with CP participated (26 males, 15 females; mean age 13y 7mo, SD 1y 8mo, range 11–17y; Gross Motor Function Classification System levels: I, n=21; II, n=17; III, n=3; unilateral distribution n=16, bilateral n=25). Primary outcome was physical activity (NL-1000 accelerometers and self-report [Multimedia Activity Recall for Children and Adolescents: MARCA]). Secondary outcomes were exercise knowledge (a purpose-designed scale), attitudes, intention and self-efficacy (Lifestyle Education for Activity Program II scales), self-reported sedentary behaviour (MARCA), and functional capacity (6-min walk test).

Results  At 10 weeks the intervention group showed no increased physical activity compared with the comparison group (weekly steps: change of +2420 vs −12189 steps p=0.06; weekly moderate-to-vigorous physical activity: change of +70 vs +8min, p=0.06; weekly distance walked: change of +3 vs −9.1km, p=0.05) and exercise knowledge (12% vs 1% improvement, p=0.08). There were no statistically significant differences for these outcomes at 20 weeks, or in self-reported physical activity at 10 or 20 weeks.

Interpretation  There was a positive short-term pattern for improvement in physical activity and knowledge. Internet-based programs may offer an alternative for participants unable to attend regular face-to-face physical activity programs.