In the earliest days of the NBA, three players entered the NBA without having played in college (although one of them did not enter the league until he was 39 years old). However, the league eventually established a rule that "a player could not make himself available" for the draft until two years after his high school graduation.
The first major challenge to the NBA's eligibility rules came from Spencer Haywood. He graduated from high school in 1968, at a time when college freshmen were not allowed to play varsity sports for NCAA member schools. He played three years at a Colorado junior college, followed by a season at the University of Detroit. After the 1970–71 season, he left college for the NBA's rival at the time, the ABA, which had no rule restricting college underclassmen from entering the league, and had a spectacularly successful rookie season with the Denver Rockets (the predecessor to today's Denver Nuggets), being named the ABA's Rookie of the Year and MVP. Near the end of the season, he turned 21; shortly after its end, he repudiated his contract with the Rockets, claiming he had been defrauded. Haywood then signed a contract with the now defunct Seattle SuperSonics, which put him and the Sonics on a collision course with the NBA, as he was only three years removed from his high school graduation.
The NBA threatened to disallow the contract and impose sanctions against the Sonics. Haywood responded by filing an antitrust suit against the league, seeking an injunction to prevent the NBA from disallowing the contract or punishing the Sonics. The case eventually reached the U.S. Supreme Court, which issued a 7–2 decision in Haywood's favor in 1971.
After the decision, the NBA allowed players to leave college early as "hardship cases", which essentially meant that the player had to prove financial hardship. This rule quickly developed into one that was observed in the breach, with Sport magazine writer Jackie Lapin commenting in the 1970s that "Almost anyone who has been any good at the game in the past decade would qualify — with the probable exception of Bill Bradley, the banker's son."
As a consequence of the aforementioned Haywood decision, and following soon after, three high schoolers chose to enter the professional ranks without ever enrolling in a college. The first was Moses Malone, who went to the ABA upon his high school graduation in 1974, almost immediately establishing himself as a star of the future. After the ABA–NBA merger in 1976, his career continued on its upward trajectory, ultimately earning him three NBA MVP awards, four appearances on the All-NBA First Team, 12 consecutive NBA All-Star Game appearances, an NBA title, a place among the 50 Greatest Players in NBA History, and enshrinement in the Naismith Memorial Basketball Hall of Fame. In 1975, Darryl Dawkins and Bill Willoughby both went to the NBA from high school. Dawkins had a solid 14-year career in the NBA, while Willoughby was no more than a journeyman in eight NBA seasons.