With no championship points in Formula One and no prospects to drive for a competitive team, Guerrero left at the end of the 1983 season to race in the United States. He had an auspicious beginning to his Indycar racing career, winning both CART and Indianapolis 500 rookie of the year honors in 1984. His initial promise was never completely fulfilled, winning only two CART races, both in 1987. Later the same year he had a massive accident which left him in a coma for 17 days.