Objective: To evaluate the feasibility and effectiveness of a 3-month minimal physical activity (PA) intervention in adolescents. Methods: A randomised controlled trial, including five secondary schools (n=87). In the 3-month intervention (Amsterdam, The Netherlands, 2005) adolescents were provided with a PAM accelerometer, coupled to a web-based tailored PA advice (PAM COACH). Measurements (i.e., PA, determinants of PA, aerobic fitness and anthropometrics) took place at baseline and at 3- and 8-month follow-up. Results: Sixty-five percent of the participants in the intervention group reported to have worn the PAM frequently and 56% of the PAM users uploaded their PAM scores to the PAM COACH at least once. We found significant differences between groups in favour of the intervention group in moderate intensity PA (MPA) for girls after 3. months (411. min/week; 95% CI: 1; 824; . P=0.04) and in sedentary time for boys after 8. months (-1801. min/week; 95% CI: -3545; -57; . P=0.04). Conclusions: Although the process evaluation suggests that a substantial proportion of the participants did not regularly wear the PAM and did not upload information to the PAM COACH website, our findings suggest promising intervention effects on MPA among girls and sedentary time among boys. © 2010 Elsevier B.V.