X started serialization in Kadokawa Shoten's Monthly Asuka on May 1992. Publishing the series proved troublesome on account of its subject matter. Publishing the series proved troublesome on account of its disasters it shown which were reminiscent of ones that happened in Japan such as earthquakes or murders committed by juveniles. Serialization stopped in March 2003 and in March 2005 Clamp stated they were searching for a proper magazine to conclude it. The chapters have been collected in a total of eighteen tankōbon volumes with the first one released on July 29, 1992 and the eighteenth on September 17, 2002. On September 26, 2006, Kadokawa Shoten published Clamp Newtype Platinum, a special Clamp edition of Newtype Magazine. The issue includes the "X 18.5" supplement, a re-print of five previously uncollected chapters. The series is expected to reach twenty-one volumes upon completion.
The North American version of the manga, retitled X/1999 in its initial printing, was serialized in Viz Media's Animerica Extra and released in form under the Shōjo imprint. In July 2001, Viz Media removed the series from Animerica Extra due to licensing issues, but it later returned in its issue published in March 2003. All the eighteen volumes have been released by Viz Media as of May 10, 2005. Beginning in 2011 Viz is re-issuing the series as a series of three-in-one volumes, unflipped, and now titled X in North America.