Cook auditioned for American Idol in Omaha, Nebraska, performing Bon Jovi's "Livin' on a Prayer." He originally did not plan to try out for the show; he came to the auditions initially to support his younger brother Andrew (who did not make it to Hollywood) and then decided to try out himself. For his first Hollywood audition, Cook performed "(Everything I Do) I Do It for You" by Bryan Adams, accompanying himself on acoustic guitar. Later in the week he performed "I'll Be" by Edwin McCain.
Cook has taken advantage of the decision to allow contestants to play musical instruments. Besides his Hollywood audition, he also accompanied himself on electric guitar for his performances of "All Right Now," "Hello," "Day Tripper," "I'm Alive," and "Baba O'Riley" and on acoustic guitar for "Little Sparrow" and "All I Really Need Is You"." His white, left-handed Gibson Les Paul electric guitar has the letters "AC" on it; as Cook told TV Guide, "I have two brothers, Adam and Andrew. So, because of superstition, I put their initials on everything growing up." Since the Top 12 week, he has also been wearing an orange wristband to support a 7-year-old fan, Lindsey Rose, with leukemia.
Cook has been both praised and criticized for his song choices, as several of his performances have been based on covers of the original songs. "Billie Jean," one of his most notable performances so far, was credited on the show to Chris Cornell and was highly praised by all three judges, especially Simon Cowell (Cowell later said in an interview with Entertainment Weekly that the performance "was in a different league to anything we've heard on the season so far," and although he knew where the arrangement came from, "that doesn't really matter.") The performance received great attention on YouTube with over 1 million views. Another of Cook's performances, "Day Tripper," was credited to Whitesnake. It has been claimed that his performance of "Eleanor Rigby" was based on Seattle-based band Doxology's version. On April 1, before performing his self-arranged rendition of "Little Sparrow" on American Idol, Cook responded to Ryan Seacrest in the interview session, revealing that his performance of "Eleanor Rigby" was based on Neil Zaza's and Doxology's versions. He also reiterated the credits of Whitesnake and Chris Cornell. Despite the controversy, critics praised Cook for choosing versions of songs that fit his vocal style. His arrangements of "Hello," "Little Sparrow," "Always Be My Baby," and "All I Really Need Is You" were original arrangements.
On April 1, 2008, a few hours after American Idol aired, TMZ.com first reported that Cook was driven to the hospital in a rush because his "heart was palpitating at a high rate and his blood pressure was high." The website claimed that Cook was not feeling well before the performance of Dolly Parton's "Little Sparrow" that night, but he had refused to go to the emergency room until the end of the show. Stress over a recent "setback" in his brother Adam's struggle with cancer may have been a contributing factor. Subsequently, David Cook was released from the hospital and is so far doing fine.