In 1939, that organization merged into the United States Chess Federation (USCF) and the tournament became the U.S. Open.
The 1963 Open at Chicago had 266 entries, making it the largest chess tournament held in the United States to that time.
The 1983 Open at Pasadena was the largest ever, at 836 official entries; it also featured the participation of Viktor Korchnoi, who had played in the last two World Championship matches.
The tournament began in some editions to decline both in attendance and importance, and often many winners are involved in a tie for first, especially since the mid 1990s.
The cash prizes awarded were large for their time and added to the tournament's popularity.