The relationship between tobacco and illicit drug use was examined among 166 methadone-maintained persons participating in a smoking cessation intervention. Latent-growth latent-variable models showed a significant relationship during treatment between rates of change in heroin and rates of change in tobacco use, with increased heroin use corresponding with increased tobacco use. Although levels of cocaine use were related to levels of tobacco use, there was no significant relationship between the rates of change of the 2 substances. A more traditional longitudinal structural equation model demonstrated a significant relationship between more heroin use during treatment and negative smoking cessation outcomes at 6-month follow-up. Findings demonstrate the utility of latent-growth models for analyzing short-term clinical trial data and strongly suggest that successful smoking cessation in this population requires a concurrent focus on reducing heroin use.