In 1989, the government of Mexico City introduced a program, Hoy No Circula, that bans most drivers from using their vehicles one weekday per week on the basis of the last digit of the vehicle's license plate. This article measures the effect of the driving restrictions on air quality using high-frequency measures from monitoring stations. Across pollutants and specifications there is no evidence that the restrictions have improved air quality. Evidence from additional sources indicates that the restrictions led to an increase in the total number of vehicles in circulation as well as a change in composition toward high-emissions vehicles.