The investigators will study the effects of an inhospital exercise intervention combined with lifestyle--including diet--counselling along the duration of treatment (neoadjuvant \[solid tumours\]/intense chemotherapy \[leukemias\], expected median duration 5-6 months) on several health-related variables. Participants will be recruited from 4 hospitals in Madrid (Spain). Inclusion criteria: male/female aged 12-19 years, newly diagnosed with a malignant extracranial tumour; not having received any type of therapy--except surgery--at the time of diagnosis; adequate health status (Karnofsky/Eastern Cooperative Oncology Group scale score ≥50/2); to understand Spanish language and to provide written informed consent. The investigators will recruit ≥136 participants and conduct a randomised controlled trial. In addition to usual care, the control group will be informed of the benefits of a healthy lifestyle. The intervention group will follow a physical exercise and lifestyle counselling program. The exercise intervention will be performed in the hospital gymnasium, except for neutropenic phases--during which time sessions will be performed in the patients' ward--and will also include inspiratory muscle training. Health counselling will include a psychological intervention based on motivational interviewing techniques, guidance by a nutritionist, and support sessions with survivors who will share their experiences with the study participants. The following outcomes will be assessed at baseline (diagnosis), end of treatment, and at 3-month follow-up in all participants: echocardiography-determined left ventricular function (primary outcome); and other echocardiography-determined variables, cardiac biomarkers, blood pressure, blood lipids, body composition, physical activity levels, energy intake, cardiorespiratory fitness, muscle strength, functional mobility, health-related quality of life, cancer-related fatigue, clinical variables, and potential biological underpinnings of exercise multisystemic benefits (metabolic and inflammatory markers, plasma proteome, gut microbiome, and immune function).