WATCH VIDEO Justin Bieber vomits on stage in Arizona
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The singer was in the middle of upbeat track Out Of Town Girl when he buckled over and violently vomited on stage, right in front of his fans.
But not even this would put them off - after running off and apologising to the crowd at the Jobing Arena, they cheered him in support.
Justin asked: "Will you love me even though I’m throwing up?"
Justin had to leave the stage twice because of his illness, but the concert - the first of his new Believe tour - went on.
He wrote on Twitter at the end of the evening: "Great show. Getting better for tomorrow's show !!!! Love u."
He posted a photo of himself lying in bed, then blamed his nausea on drinking too much milk.
The 18-year-old added: "And .... Milk was a bad choice! Lol."
Justin last night took the tour to Las Vegas, determined not to disappoint his fans.
One Belieber wrote on Twitter: "Saw that Bieber threw up on stage and had the flu yesterday at his concert in AZ and still performed today in Vegas #respect #PulledAGame6."
Justin dedicated his Arizona show to his late young fan Avalanna Routh, who died last week after a battle with cancer.
He wrote: "Goodnight Avalanna. Tonight was for u. I love u."
Justin first met six-year-old Avalanna - or "Mrs. Bieber", as she liked to be called - on Valentine's Day.
He flew to New York for a special date with her and her mum following an online campaign called "Get Avalanna to Meet Justin Bieber".
Justin was understandably devastated to hear of Avalanna's passing.
He tweeted last week: "Just got the worst news ever. one of the greatest spirits i have ever known is gone. please pray for her family and for her."
He added: "RIP Avalanna. i love you."
Watch Justin throw up on stage (if you so wish!) below...