OBJECTIVE: To study for the first time in a randomized controlled trial the feasibility, safety and efficacy of an exercise training programme in children and adolescents with JDM. METHODS: Patients were randomly assigned to the Intervention Group (IG; n = 14) or Waiting Control Group (WCG; n = 12). The intervention comprised an individually tailored 12-week home-based exercise programme of treadmill interval training and strength exercises. The efficacy of the IG over usual care (WCG) was examined with mixed linear regression (intention-to-treat). Effect sustainability during 12 weeks follow-up was also examined. RESULTS: Seventy-five percent of the participants completed the intervention. Reasons for discontinuation were motivation/fatigue, recurrent infections and increasing physical complaints. No hospitalizations occurred and immune suppressive therapy remained stable or decreased in the patients who participated in the intervention. The estimated marginal means after the intervention period were significantly in favour of the IG compared with the WCG for standing long jump distance [difference between groups (95% CI): 13 cm (2-23)], the 30-s number of push-ups [8 (3-13)] and sit-ups [4 (0.4-8)], and the parent Childhood Health Assessment Questionnaire 30 + 8 score [-0.13 (-0.24 to - 0.01)] and effects sustained at follow-up. A trend was seen for the maximal oxygen uptake divided by body mass during maximal exercise treadmill testing; the IG scored 3.0 ml/kg/min (-1.3 to 7.3) higher compared with the WCG. Other outcomes (e.g. isometric muscle strength and perception of fatigue) did not differ between IG and WCG. CONCLUSION: Exercise training is of value in the clinical management of JDM.