David Cook couldn't compose himself as he performed "Always Be My Baby" in front of his ailing brother, Adam, on Tuesday's American Idol.

David, 25, had tears in his eyes as he sang the Mariah Carey tune as Adam, 36 — who is battling brain cancer — watched on.

As soon as David started singing, Adam — his IV still hanging from his sleeve — broke out "in a big grin and didn't stop smiling for a while," an on-looker tells reporters.

Despite his frail appearance, Adam (who was seated next to Teri Hatcher) stood up and clapped.

Cowell praised David's performance.

“You have tears,” host Ryan Seacrest told David.

“Yeah,” mustered David, who was recently hospitalized for high blood pressure and heart palpitations.

During the break, David ran into the audience and rubbed his brother's back. Judges Randy Jackson and Paula Abdul then hugged David.

"It was an intense moment," an on-looker tells Us. "Everyone seemed a little overwhelmed."