This study examines the association between substance use and crime by modeling change within subjects over an 11 month period in a sample of 157 chronic drug-using offenders. For this sample, increased substance use—cocaine or heroin use as well as alcohol use—was significantly related to increases in self-reports of income generating but not violent crime. The study also demonstrates a significant effect of drug treatment in the last month on income generating crime, but not on violent crime and that the effect of drug treatment on income generating crime is mediated by reductions in drug use. This work refines prior work by showing that drug use effects vary by crime type and by providing further evidence that drug treatment reduces cocaine and heroin use, which leads to a reduction in property crime. It is the first study to examine variability over time in all three components (drug treatment, drug use, and crime) while adequately controlling for individual level propensity variables.