In 2362 Lieutenant Leijten was part of an away team with La Forge along with three other crewpersons that landed on the planet Tarchannen III. They were exposed to an alien retro-DNA which took over three members of her former crew and turned them into alien lifeforms.
Between 2362 and 2367 she almost married an old aquaintance of her, Paul Bogrow.
In 2367 she arrived on the USS Enterprise-D to inform La Forge that they 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. She helped in the rescue of La Forge, who also was mutating and had returned to Tarchannen III. (TNG: "Identity Crisis")