X-Men: The Hidden Years

The series attempted to fill in the period in the late 1960s and early 1970s when the original X-Men comic was publishing only reprints of earlier issues. It features the cast of the original X-Men and their villains, with a few appearances by characters who had not otherwise appeared at that point in time, such as Storm.

The series was originally conceived to run 27 issues, one issue for every reprint issue of the original series (#67-93), but was cancelled after only 17 issues, and Byrne had stated that he had ideas that might have extended the series.

Marvel's editor-in-chief at the time, Joe Quesada, cited the series as one of the more extraneous X-Men spinoffs, and cancelled it, along with several other X-Men franchise titles, in an effort to streamline Marvel's output. It is believed that part of the reason for the cancelation was because Marvel wanted to also simplify the continuity of the X-Men, and did not want a series around that dealt with the "hidden" time of issues that were published in the 1970s.

Byrne had claimed that the comic was still profitable, and this cancellation along with a lack of explanation why it was done, was a percipitating factor for him to decide to no longer work for Marvel Comics. Notably, Byrne's X-Men title was one of the lowest selling X-Men titles at the time of its publishing, ranking far below other titles such as Uncanny and even some of the tetriary spin-offs of the time, so it was likely due for cancellation.