Joe Jonas checked into an emergency room in L.A. while doctors inspected his abs -- for severe stomach pains -- 48 hours after Joe did some late night partying in Chicago.

A rep for Joe confirms he went to the emergency room at Children's Hospital L.A. on Sunday. We're told Joe was only at the hospital briefly ... and it's not clear what was wrong with him.

One potential hint -- on Friday night (into Saturday morning) Joe was out at a Chicago hot spot Board Room Nightclub (above) partying ... and performed the next night in Chi-Town.

Whatever the problem was ... Joe felt good enough to eat out -- we spotted him and his bro Nick leaving Katsuya last night. Sushi after stomach troubles? Risky, Joe.

Check out some beachin' celebrity doppelgangers -- and vote for which star you think owns the look!

Wearing a vinyl sauna sweat top, newly single Jonas Brother Joe Jonas worked on his fitness in L.A. on Monday.



He's a lean, mean, boy band machine.

"Twilight" star Ashley Greene just broke up with Joe Jonas and has already been seen out in public with Kings of Leon bassist Jared Followill.


Question is ...

To celebrate her 24th birthday, "Twilight" hottie Ashley Greene got real cozy with her younger boyfriend Joe Jonas, 21, at PURE in Vegas this weekend.


It may not be his birthday, but it looks like Joe's the one who got the best present.

Disney dudes Zac Efron, 23, and Joe Jonas, 21, put on their Sunday best at a fashion event in NYC this weekend.


Question is ...

Boy bander extraordinaire Joe Jonas, 21, and Olympian Shaun White, 24, both rocked out at a concert in L.A. on Sunday.


Question is ...

Joe Jonas, 21, Kevin Jonas, 22, and Nick Jonas, 18, all flexed their evangelical Christian dandy boy band muscles at the same event in Costa Rica on Tuesday.



Question is ...

Joe Jonas is prepared to go to war over a blog posting that insists he and his underage ex-girlfriend Demi Lovato are currently "friends with benefits" ... in fact, Joe's lawyers are already on the attack.



TMZ has learned ... Joe's attorney just fired off a letter to a Canadian entertainment blogger named Zack Taylor ... blasting the guy for a story he posted which implied that 20-year-old Joe and 17-year-old Demi "are involved in an illicit relationship."

In the letter, obtained by TMZ, Jonas' attorney claims the story is "completely false, offensive and defamatory" -- and accuses Taylor of a "malicious attempt to attack and devalue [Joe's] good name and reputation."

Team Jonas is demanding that Taylor pull the post ASAP ... or else a lawsuit may follow.

Taylor tells TMZ, "We are currently reviewing the situation & letter. However, at this time we
do stand by our sources and their claims."

After finally confirming they are a couple, Demi Lovato and Joe Jonas stepped out in public yesterday to do a little platonic shopping in Los Feliz.



Rumors that they're the same size are not true.

Evangelical Christian dandy boy band biological brothers Kevin Jonas, 22, and Joe Jonas, 20, stayed pure at the same event in L.A. on Wednesday.



Question is ...