OBJECTIVE:To determine the implications of car ownership for physical activity and weight in a global city. DESIGN:Quasi-experimental cross sectional study. SETTING:Beijing, China, 2011-15. PARTICIPANTS:People aged 18 and older from a random sample of households who had entered a permit lottery to purchase a vehicle between January 2011 and November 2015. INTERVENTIONS:Permit allowing purchase of a vehicle within six months of permit issuance. MAIN OUTCOME MEASURES:Transit use (number of subway and bus rides each week), physical activity (minutes of walking or bicycling each day), and weight, measured once in early 2016. RESULTS:Of 937 people analysed in total, 180 had won a permit to purchase a new vehicle. Winning the permit lottery resulted in the purchase of an additional vehicle 91% of the time (95% confidence interval 89% to 94%; P<0.001). About five years after winning, winners took significantly fewer weekly transit rides (-2.9 rides (-5.1 to -0.7); P=0.01) and walked and cycled significantly less (-24.2 minutes (-40.3 to -8.1); P=0.003) than those who did not win the lottery. Average weight did not change significantly between lottery winners and losers. Among those aged 50 and older, however, winners' weight had increased relative to that of losers (10.3 kg (0.5 to 20.2); P=0.04) 5.1 years after winning. CONCLUSIONS:These data indicate that vehicle ownership in a rapidly growing global city led to long term reductions in physical activity and increase in weight. Continuing increases in car use and ownership in developing and middle income countries could adversely affect physical health and obesity rates.