The authors investigated the medical use, illicit use, and diversion of 4 distinct classes of abusable prescription medication (sleeping medication, sedative or anxiety medication, stimulant medication, and pain medication) in a random sample of undergraduate students. In spring 2003, 9,161 undergraduate students attending a large, public, midwestern research university in the United States self-administered a Web-based survey. The prevalence rate for illicit use within the past year was highest for pain medication, followed by stimulant medication, sedative or anxiety medication, and sleeping medication. Women generally reported higher past-year medical use rates. However, undergraduate men reported higher illicit use rates. The illicit use--medical use ratio for stimulant medication was the highest among the 4 classes of prescription drugs. Medical users of stimulants for attention deficit hyperactivity disorder were the most likely to be approached to divert their medication. Multivariate results indicated that illicit users of prescription drugs were more likely to use other drugs than were students who did not use prescription drugs illicitly. The authors provide evidence that prescription drug abuse is a problem among college students. (Contains 2 tables.)