Introduction and objectives: Physical fitness level is a marker of cardiovascular health in young people. The aim of this study was to analyze the effects of a school-based intervention program, focused on increasing the volume and intensity of physical education (PE) sessions, on adolescents’ physical fitness. Methods: Sixty-seven adolescents (12-14 years old) from three secondary school classes participated in a 16-week intervention. The classes were randomly allocated to the control group, experimental group 1 (EG1) or experimental group 2 (EG2). The control group received standard PE (2 sessions/week), the EG1 received 4 standard PE sessions/week and the EG2 received four high-intensity PE sessions/week. Aerobic fitness, muscle strength, speed-agility and flexibility were assessed using previously validated field-based tests before and after the intervention. Results: Doubling the number of PE sessions/week resulted in improvements in aerobic fitness and flexibility (P = .008 and P = .04, respectively). Further increases in the intensity of the sessions were related to improvements in speed-agility (P < .001). The maximal oxygen consumption increased by 3 and 5 mL/kg/min in the EG1 and EG2, respectively. No differences were observed for muscle strength. Conclusions: The results suggest that doubling the frequency of PE sessions is a sufficient stimulus to improve physical fitness, particularly aerobic fitness, which has been shown to be a powerful indicator of cardiovascular health in children and adolescents. Future studies involving larger sample sizes should confirm or refute these findings. (C) 2011 Sociedad Espanola de Cardiologia. Published by Elsevier Espana, S.L. All rights reserved.