Objective: To identify the predictors of nonmedical ADHD medication use by college students. Participants: A total of 843 undergraduates attending one public or one private university in southeastern United States. Method: Students completed a Web-based survey inquiring about ADHD medication use during the first semester freshman of their year and a second time during the second semester of their sophomore year. Results: A total of 45 participants (5.3%) reported the onset of nonmedical ADHD use between the two survey administrations. The majority of these students were high substance users as freshman. Attention difficulties also predicted the onset of nonmedical use, especially in the absence of excessive substance use. Compared with other nonmedical users of ADHD medication, those reporting attention difficulties had lower GPAs, greater academic concerns, and higher levels of depressive symptoms. Conclusion: Attention difficulties contribute to the onset of nonmedical ADHD medication use in a significant minority of nonmedical users. These students may begin using ADHD medication to address attention problems they experience as undermining their academic success. (Contains 4 tables.)