The purpose of this study was to evaluate the impact of an extra-curricular school sport programme to promote physical activity among adolescents. One hundred and sixteen students (mean age 14.2 years, s = 0.5) were assigned to an intervention (n = 50) or comparison group (n = 66). The 8-week intervention involved structured exercise activities and information sessions. Four days of pedometer monitoring and time spent in non-organized physical activity and sedentary behaviours were measured at baseline and post-test. At baseline, participants were classified using steps per day as low-active (girls <11,000, boys <13,000) or active (girls > or = 11,000, boys > or = 13,000) and the effects of the intervention were assessed using these subgroups. Adolescents in the intervention group classified as low-active at baseline increased their step counts across the 8-week intervention (baseline: 7716 steps/day, s = 1751; post-test: 10,301 steps/day, s = 4410; P < 0.05) and accumulated significantly more steps (P < 0.05) than their peers in the comparison group (baseline: 8414 steps/day, s = 2460; post-test: 8248 steps/day, s = 3674; P = 0.879). The results of the present study provide further evidence that physical activity monitoring using pedometers is an effective strategy for increasing activity among low-active adolescents.