The authors explain that cross-sectional data concerning the prevalence of substance abuse may mask differences among ethnic groups. This study represents a preliminary attempt to detangle the evidence that shows Black adolescents have lower levels of substance abuse than white adolescents, while whites generally have lower levels of substance abuse than Blacks by the time of early adulthood. The authors analyzed longitudinal data from 1,004 white, Black, and Hispanic respondents from Chicago. The goal was to describe the onset, prevalence, and cessation of substance abuse and dependence for these three groups. Two age cohorts were examined: 15 and 18 year olds made up the baseline measure, while 17 and 20 year olds provided follow-up data. Results indicated that the prevalence of alcohol and marijuana use at age 15 did not differ significantly among the three groups. However, rates of onset for alcohol abuse between the ages of 15 and 17 were significantly higher for whites than for Blacks and Hispanics. Rates of onset for marijuana abuse between the ages of 18 and 20 were significantly higher for Blacks than for whites and Hispanics. There were no significant differences among the three groups for persistence of drug abuse. In conclusion, the authors show that between adolescence and young adulthood rates of substance abuse change. The consequences are especially pronounced among Blacks, as their rates of abuse climb upward while the rates for whites and Hispanics show declines. In conclusion, the authors call for more longitudinal research regarding the etiology of substance abuse and dependence among different ethnic groups. Tables, references