Representative data from the U.S. National Health Interview Survey are used to investigate the substance use patterns of immigrants and compare them to those of the native born populations. The information examined is from the 1991 supplementary Drug and Alcohol Use Data File, which examined the self-reported substance use behaviors of approximately 21,000 adults aged 18-44. Findings indicate that immigrants to the U.S. in the late twentieth century are less likely to use alcohol and other drugs than are native born citizens. Additional findings suggest assimilation processes by which exposure to mainstream American society leads to patterns of alcohol and illicit drug use among long term immigrants that approximates that of the native born population. The patterns of substance use observed among immigrants, however, are not consistent with acculturative stress mechanisms. These findings provide an important and representative profile of the substance use patterns of one of the largest international migration streams of the past one hundred years.