Rich Cronin from the '90s boy band LFO died today after a long fight with leukemia ... TMZ has learned

Rich -- who wrote LFO's hit "Summer Girls" in 1999 -- suffered a stroke after a lengthy battle with leukemia, according to his brother. We're told he passed away in a hospital this afternoon.

Cronin was 35

http://www.tmz.com/

:rip: Rich Cronin

http://i51.tinypic.com/2lk7437.jpg

That's horrible. He was way too young. I saw LFO open for Britney Spears 10 1/2 years ago.

:rip:

:rip:

So sad to hear the news of Rich's passing... he was so young... :(