AbstractTraditionally, researchers and clinicians have viewed street heroin users as representing an end point in a drug-using career. It is toward this population that major national treatment efforts have been directed. Concomitant with the historical national concern about heroin use has been concern about the increase in crack cocaine use. There has been speculation that crack use may have become a substitute for the higher risk of heroin injection. This article examines the impact of crack use in a national population of heroin users. It was consistently found that there was a correlation between the frequency of crack use and the frequency of speedballing as well as the use of other forms of cocaine, amphetamines, and marijuana. There was also a positive relationship between frequency of crack use and such HIV risk behaviors as renting needles and having a large number of sex partners. The data suggest that in this population of heroin users, crack is not a substitute for heroin but rather is a part of a drug-use pattern that includes an increased use of other drugs and increased high-risk behaviors for contracting HIV.