Although cigarette smoking is endemic among illicit drug users, drug abuse treatment programs rarely encourage smoking cessation and often discourage it. The purpose of this study was to determine whether smoking cessation after entering drug abuse treatment influenced drug use 12 months after drug abuse treatment. We analyzed 2316 cigarette smokers in the Drug Abuse Treatment Outcome Study (DATOS), a national, longitudinal study of drug abuse treatment. Heckman probit selection models assessed the association of self-reported smoking cessation while in drug abuse treatment on self-reported drug abstinence in the year after treatment completion, while simultaneously accounting for possible nonparticipation bias. Controlling for multiple factors, smoking cessation was associated with greater abstinence from drug use after completion of drug abuse treatment (P=.04). Despite drug abuse treatment programs' hesitance to encourage smokers to quit, smoking cessation does not negatively impact drug use outcomes.