Prior research has consistently demonstrated an association between substance use and involvement in violence among individuals with mental illness. Yet little is known about the temporal quality of this relationship, largely because longitudinal data required to address this issue are not readily available. This study examined the relationship between substance use (alcohol, marijuana, and other drug use) and violence at the daily level within a sample of mentally ill individuals at high risk for frequent involvement in violence (N = 132). Results support the serial nature of substance use and violence, with an increased likelihood of violence on days following the use of alcohol or multiple drugs, but not the inverse relationship. Implications for the utility of substance use as a risk marker for the assessment of future violence are discussed.