Recent work has shown that a substantial proportion of injecting drug users would have met criteria for antisocial personality disorder (ASPD) if the childhood trajectory of conduct disorder (CD) were ignored. From among 545 St. Louis, Missouri, drug users interviewed in person, we evaluated the clinical homogeneity of the 405 men and women with adult antisocial behaviors who did and did not have conduct disorder. The fully diagnosed ASPD group (those with CD) was distinguishable from the adult antisocial behavior only group without CD on all childhood behaviors, adult impulsive and aggressive behaviors, and measures of severe drug abuse, including earlier age of onset of drug use and drug treatment utilization. Clinical homogeneity of the groups was reflected by the group's similarity on indices such as types of adult antisocial behaviors, consequences of drug use, injection drug use history, and comorbid psychiatric disorders. Implications for a modification in the concept of ASPD are addressed.