A renewed American interest in marijuana has coincided with our research group's focus on the earliest stages of drug involvement. Here, we have studied the transition from an initial opportunity to try marijuana to the subsequent use of this drug. We analyzed self-report interview data gathered from nationally representative samples of the United States National Household Surveys on Drug Abuse, 1979-1994. The evidence indicates that the estimated prevalence of an opportunity to try marijuana has been rather stable for 15 years. However, there are recent increases in the probability of rapidly progressing from first marijuana opportunity to first marijuana use, among persons given an opportunity to use. In addition, the transition from first marijuana opportunity to eventual marijuana use seems to depend upon age at first opportunity. This epidemiological evidence on the transition from marijuana opportunity to marijuana use, the first to be published based on a nationally representative US sample, highlights directions for future research and a focus for prevention efforts.