Late in Season 4 (https://www6.fmovies.to/film/hill-street-blues-4.6lv0p/6189n4), LaRue (Kiel Martin) simply disappears for about four episodes, with one reference to him being sick and then it being "mono, so expect him to be out for a few weeks." He remained in the lead credits, appears in the season finale and of course continued to the end of the show.

Detective Harry Garibaldi (Ken Olin) is propped up in his place and becomes a continuing character, but seems very much like a cardboard cutout in these shows and delivers a lot of dialogue that sounds like it was written for LaRue.

What happened to Martin in these weeks? Bare in mind that he did nothing (https://www.imdb.com/name/nm0552633/) but Hill Street Blues until its conclusion (https://www.sun-sentinel.com/news/fl-xpm-1988-04-05-8801210716-story.html) in 1987.

Kiel Martin struggled with drug addiction at that time in his life. Wouldn’t surprise me if this caused absences from the show. Although it was reported that he died of lung cancer, other sources said it was complications due to AIDS, contracted from a needle. How sad.

The end of season 4 without Kiel/LaRue is just a boring rewatch for me as he was my fave.

So, what's this about him having AIDS and dying of that because I have never heard that before.

Somebody on Facebook recently asked a similar question (https://www.facebook.com/groups/hillstblues/posts/5207818252573266/):
Anyone know why Kiel Martin was absent from several episodes at the tail end of Season 4? The show explains it as mono (what we in the UK call glandular fever).
I did notice that in a few of the episodes before that, his voice seems to go noticeably more raspy - he almost sounded like Harvey Fierstein! So perhaps it was a real-life illness.

I am a big fan of Hill Street since it first came on in the 1980's when I was in high school. I will watch the whole series again about once a year.

From what I have read, Kiel Martin wasnt that much different than his character LaRue. Like LaRue, Martin was an alcoholic and a cocaine abuser. I have also read from Steven Bochco than Martin was a pain in the ass and not always reliable and when he was there would sometimes goof off and pull practical jokes. Martin was LaRue and LaRue was Martin. Sometimes an actor of a role is very similar to the person, which makes them much easier to play.

But for Martin to die of lung cancer at 46, he must have smoked a lot or was just unlucky.