In professional cycling, stage races like the three tours, Italy, Spain, France, the blood doping that Lance Armstrong did made a huge difference and gave him a ridiculous advantage. On mountain stages, he would just pedal away from the other leaders like he had a motor on his bike. I realize other cyclists during that period were doping as well, but not to the extent or in the way that Armstrong was.
I remember Greg Lemond said that in his last two Tours, he felt a big difference in the peleton than earlier years. Whereas, the early flat stages of the Tour were traditionally used to fine-tune fitness and to ease into the three week race, he saw riders at full speed right from the first few days. He attributed it to blood doping, which increases oxygen levels, vital in endurance competitions. Lemond eventually became a critic of Armstrong and publicly offered data to support his claim that the guy was cheating. Lemond was vilified, but in the end was proven right. He remains the lone American to win the Tour de France ('86, '89' 90).