This randomized experiment examined survey mode effects for self-reporting illicit drug use by comparing prevalence estimates between a Web-based survey and a mail-based survey. A random sample of 7,000 traditional-aged undergraduate students attending a large public university in the United States was selected to participate in the spring of 2001. Students were randomly assigned to self-administer a survey via the Web or U.S. mail. The Web survey produced a significantly higher response rate than the U.S. mail survey. The prevalence estimates of illicit drug use (lifetime and past year) did not differ significantly between the two survey modes. The findings provide preliminary evidence that Web and U.S. mail surveys produce similar results regarding illicit drug use among undergraduate students. Although additional research is needed involving more diverse samples, these findings bode well for using Web surveys in college-based research.