Robert Vaughn worked on his Ph.D. during the course of the series and often was allowed to leave the set early so that he could attend night classes.
The show's third season saw a change of style that resulted in the amount of comedy being increased in response to the "camp" craze made popular by "Batman (1966)" and "Get Smart (1965)." As a result, this show's ratings plummeted and the series never recovered. Despite a return to serious stories the next year, it was cancelled midway through its fourth season.
Ian Fleming was reportedly one of the consultants when this series was being planned. The name Solo was borrowed, apparently with his blessing, from the novel "Goldfinger."
During the series' run, Illya became so popular that David McCallum received more fan mail than any other actor in MGM's history, including stars like Clark Gable and Elvis Presley.
The meaning of the acronym T.H.R.U.S.H. was never spelled out in the series, though a meaning was created for one of the U.N.C.L.E. novels published at the time ("Technological Hierarchy for the Removal of Undesirables and the Subjugation of Humanity").