The objective of this study was to examine the prevalence and factors associated with the illicit use of prescription stimulants and to assess the relationship between the medical and illicit use of prescription stimulants among undergraduate college students. A Web survey was self-administered by a random sample of 9,161 undergraduate students attending a large public midwestern university in the spring of 2003. A total of 8.1% reported lifetime illicit use of prescription stimulants and 5.4% reported past year illicit use. The number of undergraduate students who reported illicit use of prescription stimulants exceeded the number of students who reported medical use of prescription stimulants for ADHD. The leading sources of prescription stimulants for illicit use were friends and peers. Multivariate logistic regression analyses revealed several risk factors for illicit use of prescription stimulants such as being male, White, member of a social fraternity or sorority, Jewish religious affiliation, and lower grade point average. All of these characteristics were also related to medically prescribed use of prescription stimulants. Those who initiated medically prescribed use of prescription stimulants for ADHD in elementary school were generally not at increased risk for illicit use of prescription stimulants or other drugs during college as compared to those who were never prescribed stimulant medication. The present study provides evidence that the illicit use of prescription stimulants is a problem among undergraduate college students, and certain subgroups appear to be at heightened risk.