To measure the levels and patterns of physical activity, using accelerometers, of 11- year- old children participating in the Avon Longitudinal Study of Parents and Children ( ALSPAC). Design: Cross- sectional analysis. Setting: ALSPAC is a birth cohort study located in the former county of Avon, in the southwest of England. This study used data collected when the children were 11 years old. Participants: 5595 children ( 2662 boys, 2933 girls). The children are the offspring of women recruited to a birth cohort study during 1991 - 2. The median age ( 95% CI) of the children is now 11.8 ( 11.6 to 11.9) years. Methods: Physical activity was measured over a maximum of 7 consecutive days using the MTI Actigraph accelerometer. Main outcome measures: Level and pattern of physical activity. Results: The median physical activity level was 580 counts/ min. Boys were more active than girls ( median ( IQR) 644 ( 528 - 772) counts/ min vs 529 ( 444 - 638) counts/ min, respectively). Only 2.5% ( 95% CI 2.1% to 2.9%) of children ( boys 5.1% ( 95% CI 4.3% to 6.0%), girls 0.4% ( 95% CI 0.2% to 0.7%) met current internationally recognised recommendations for physical activity. Children were most active in summer and least active in winter ( difference = 108 counts/ min). Both the mother and partner's education level were inversely associated with activity level ( p for trend,< 0.001 ( both mother and partner)). The association was lost for mother's education ( p for trend = 0.07) and attenuated for partner's education ( p for trend = 0.02), after adjustment for age, sex, season, maternal age and social class. Conclusions: A large majority of children are insufficiently active, according to current recommended levels for health.