Relationships were explored among the frequencies of use of various drugs by a sample of drug-abusing clients of the Addiction Research Foundation (ARF) in Toronto and by drug abusers volunteering to participate in research at the Addiction Research Center (ARC) in Baltimore. The two groups of drug-abusing individuals differed in a number of characteristics. Those from ARF were admitted primarily for diagnosis and possible treatment for alcohol and non-opioid drug problems, whereas those from the ARC were admitted for participation in research on other drugs of abuse, primarily involving opioids. Patterns of use of certain drugs tended to covary in both groups. Of particular interest was the finding that severity of alcoholism was directly related to various measures of tobacco and caffeinated beverage use. In contrast, there was little correlation between the frequency of use among other drugs of abuse (e.g., heroin, cannabis, glue) and the use of tobacco and caffeine. These findings suggest that dependence on nicotine, caffeine, and alcohol may be governed by the same factors and possibly should be considered jointly in the treatment of alcoholic persons. Frequency of use of other drugs examined may be controlled by other factors than those which determine level of use of tobacco and caffeine.