Since doping improves athletic performance, anti-doping policies should have the opposite effect. This analysis examined whether changes in the speed of major cycling races reflect recent anti-doping efforts. Average speeds of 5 (th) place finishers of the Tour de France, Giro d'Italia, and Vuelta a España cycling races were obtained for the period 1990-2009. Between 1990 and 2004, the average speed had been increasing by 0.16 km/h per year (p<0.001). In a downturn, since 2004, the average speed has decreased by 0.22 km/h per year (p=0.031). The slowing down of professional cycling races is compatible with the hypothesis that recent anti-doping efforts in professional cycling have curbed the use of performance-enhancing substances.