The leisure-time physical activity of 13-year-old Finnish adolescents was assessed in the prospective STRIP study. A self-administered questionnaire (N=565) was used. The leisure-time physical activity index (PAI; MET h/week) was calculated on the basis of reported exercise intensity, duration and frequency (N=558; 53% boys). The participants were divided into Sedentary, Moderately Active and Active groups by PAI tertiles. A subpopulation (N=197) also used a heart rate monitor (3 days, >/=8 h/day) to assess the time spent on different activity intensities. The median male PAI was 31.3 (inter-quartile range (IQR) 44.2) MET h/week and female 19.5 (IQR 26.3) MET h/week (P=0.0002). The cutoff points of the PAI tertiles were similar for the Active girls (31.3 MET h/week) and boys (32.6 MET h/week), but for the Sedentary boys it was 19.5 MET h/week and only 5.0 MET h/week for the Sedentary girls. High self-reported leisure-time physical activity associated poorly with time spent on moderate or vigorous exercise measured by heart rate monitoring. Active mothers had more often Active daughters or sons. In conclusion, the amount of leisure-time physical activity of one-third of 13-year-old girls is extremely low. Sedentary adolescents, especially girls, should therefore be put into focus for active efforts to increase their leisure-time physical activity. Parental models may be important.