In a previous study, alcohol problems in perpetrators and their partners contributed directly and indirectly to intimate partner violence (IPV), even after including other correlates of violence in the model (G. L. Stuart et al., 2006). The present study extends these findings by examining the role of illicit drug use. We recruited 271 men and 135 women arrested for IPV and used structural equation modeling to examine the data. Results showed that drug use, as reported by the perpetrators, was a stronger predictor of IPV than were alcohol problems in perpetrators and their partners. Arrested males' marijuana use and stimulant use (i.e., cocaine and amphetamines) were associated with perpetration of IPV, and their report of their female partners' stimulant use was associated with her violence perpetration. In arrested women, specific substances used did not predict violence perpetration beyond other model variables; however, female perpetrators' report of male partners' stimulant use predicted male psychological and physical aggression, after controlling for other variables. These results provide further evidence that drug problems by both partners may be important in the evolution of aggression. Implications for batterer intervention programs are discussed.