Epidemiological descriptions of drug abuse in the US in the last three decades have generally not included data on the patterns and trends in the incidence of illicit drug use (i.e. new users). In this paper, estimates of illicit drug use incidence are presented, based on retrospective data from the National Household Survey on Drug Abuse. Incidence of marijuana use began increasing in the 1960s and reached a peak in 1973, after which a continuing decline was seen. Cocaine use incidence began to increase in the late 1960s and reached a peak in 1982, then declined.