During 1979 and 1980 the JASDF received 14 unarmed RF-4E's from the U.S. These were to replace the aging RF-86F Sabres they had been using for reconnaissance. In 1982 a program was started to update the JASDF F-4EJ's by strengthening the structure, adding the in-flight re-fuelling probe, new avionics such as improved radar, new computers and wired for Sidewinders.
The upgraded aircraft were given the designation F-4EJ Kai (Kai – short for kaizen or improvement). Included in the upgrade were 13 of the 14 RF-4E that received new radars and were referred to as RF-4E Kai. These RF-4E Kai were operated by 501 Hikotai (squadron).
