Objective: To explore whether a structured counselling-based intervention increases vigorous physical activity behaviour of adolescent and young adult cancer survivors. Design: Randomized controlled phase II trial. Setting: University Cancer Center Hamburg, Germany. Subjects: Eighty-nine participants (mean age 24.1 ± 6.3) were randomized to control ( n = 44) or intervention group ( n = 45). Interventions: The intervention group was consulted about physical activity behaviour via interview (week 0), and telephone counselling (weeks 1, 3 and 12). The control group only received general physical activity guidelines for cancer survivors (week 0). Main measures: The primary outcome was the rate of participants with ⩾9 metabolic equivalent (MET)-hours per week of vigorous activity post-intervention, measured with the International Physical Activity Questionnaire. Secondary outcomes included assessing physical activity behaviour (e.g. amount and type of physical activity) and quality of life. Assessments were completed in weeks 0 (baseline), 12 (post-intervention) and 52 (follow-up). Results: Sixty-nine participants completed the post-intervention- and 47 the follow-up-assessment. The rate of participants performing vigorous physical activity increased from baseline to post-intervention for both without differing significantly ( P = 0.541). Both increased their total metabolic equivalent from baseline to post-intervention (intervention group from 55.2 ± 43.7 to 61.7 ± 29.4, control group from 75.3 ± 81.4 to 88.3 ± 80.2). At follow-up the intervention group (73.7 ± 80.2) was more active than baseline when compared to the control group (78.5 ± 50.0). Conclusions: A structured counselling-based physical activity intervention did not significantly impact the level of vigorous physical activity behaviour in adolescent and young adult cancer survivors.