In 2362, Lieutenant Leijten was part of an away team on the planet Tarchannen III with Lieutenant La Forge along with three other crew members. They were exposed to an alien retro-DNA that infected three members of Leijten's former crew and turned them into alien lifeforms.
Between 2362 and 2367, she almost married an old acquaintance, Paul Bogrow.
In 2367, Leijten arrived on the USS Enterprise-D to inform La Forge that he and herself were the only two left from the away team, and that the others had disappeared. When they landed on the planet to investigate, Lieutenant Commander Leijten started to act strangely, refusing to leave. La Forge beamed her to sickbay where Dr. Crusher found that she was covered with dark blotches and her hands were mutating. Alien DNA had started to change Leijten into an alien lifeform but she was saved by Dr. Crusher. Commander Leijten helped in the rescue of La Forge, who also was mutating and had returned to Tarchannen III. (TNG: "Identity Crisis")