In this issue of The Journal, Pope and Yurgelun-Todd1 report that frequent marijuana users, relative to less frequent users, show impairments in mental flexibility and abstraction, as well as some aspects of learning. Their study is important and timely. Following well over a decade of generally declining use, marijuana's popularity has increased markedly among US youth in the last 3 to 4 years,2 prompting renewed concerns about the drug's possible health consequences. In December 1994, US Department of Health and Human Services Secretary Donna Shalala announced a department-wide initiative on marijuana, as part of which the National Institute on Drug Abuse (NIDA) last year convened the first-ever National Conference on Marijuana Use: Prevention, Treatment, and Research. At this conference, Shalala and NIDA Director Alan Leshner emphasized that marijuana use has significant health consequences and that scientific information about the drug's dangers should be communicated to youth and parents.