The National Institute on Drug Abuse's National Cooperative Agreement Project is a nationwide study of the injection and sexual behaviors of three populations of out-of-treatment drug users: injecting drug users (IDUs) who did not smoke crack cocaine, non-injecting crack smokers, and those engaging in both types of use. The goal of the analyses we present was identification of subsets of behavioral variables that would, within each type of drug use, distinguish sites by HIV seroprevalence level. Among non-crack-smoking IDUs high seroprevalence rates were associated with race and sex, the overall frequency of injection, the frequency of cocaine injection, and geographic location. Among noninjecting crack smokers, cities were distinguished by the frequency of exchanging sex for money, the number of sex partners, the local seroprevalence rate among IDUs, and location. Among subjects engaging in both types of use, seroprevalence rates among sites was distinguished by the frequency of exchanging sex for drugs and location.