Objectives To compare cardiovascular fitness and physical activity of schoolchildren 18 months after London 2012 according to Olympic ‘inspiration’. Design A cross-sectional comparison between groups of schoolchildren categorised according to self-reported Olympic inspiration and a repeated cross-sectional comparison using data collected pre-2012. Setting Schools within a 50 km radius of the Olympic Park, Stratford, London. Participants 931 students (10.0–15.9-year-olds) attending 6 schools assessed in 2013 and 2014 (18 (range: 14–20) months after London 2012) and 733 students from the same schools assessed in 2008–2009 (42 (range: 38–46) months before London 2012). Primary outcome measures Self-reported Olympic inspiration; cardiorespiratory fitness (Graphic mL/kg/min) assessed using the 20 m shuttle-run and self-reported physical activity. Secondary outcomes measures Differences in Graphic before and after London 2012. Results 53% of children reported being inspired to try new sports or activities. Compared with those not inspired by the Games, Graphic was higher in boys (d=0.43) and girls (d=0.27), who continued to participate in activities at 18(14–20) months. This 45% of sample was also more physically active (boys, d=0.23; girls, d=0.38) than those not or only briefly inspired to participate in activities (boys, d=0.24; girls, d=0.21). Compared with pre-2012 values, Graphic was lower post-2012 in boys (d=0.37) and in girls (d=0.38). Conclusions High levels of inspiration to participate in new activities reported following London 2012 and positive associations with fitness are encouraging. We cannot discount the possibility that inspired participants may have already been fitter and more active pre-2012. These associations must be interpreted in the context of the significant declines in fitness shown by our repeated cross-sectional comparison. Olympic host countries should employ longitudinal monitoring using objectively measured fitness and physical activity to provide evidence of health-related legacy.