The aim of this pilot study was to characterize seasonal variationin the moderate to vigorous physical activity (MVPA) and sedentarybehavior of Portuguese school youth, and understand theinfluence of activity choices and settings. The participants inthis study were 24 students, aged 10-13 years. Accelerometersmeasured daily PA over 7 consecutive days, in different seasonsMay - June and January - February. In summer, boys accumulatedmore minutes in MVPA (928 minutes/week) than girls(793 minutes/week). In winter the pattern was reversed withgirls accumulating more activity than boys (736 minutes/weekvs. 598 minutes/week). The repeated measures ANOVA revealedsignificant effects for season (F = 5.98, p = 0.023) and inschoolvs. out-of-school (F = 6.53, p = 0.018). Youth were moreactive in the summer and activity levels were higher after schoolthan in school. Summer season provided relevant contexts foryouth physical activity accumulation. Winter season may havebeen a significant barrier to boy's preferred PA context. Differencesin choices of outdoor or indoor PA, after school, explainedthe gender differences in seasonal activity patterns.