Prisonization (as measured by number of prior incarcerations) and concurrent instrumental offending (as measured by contemporaneous kidnapping, rape, robbery, and burglary offenses) were found to interact in 160 multiple-homicide offenders and 494 single-homicide offenders. Controlling for age, gender, race, criminal history, prior incarcerations, and instrumental contemporaneous offending, the interaction between prior incarceration and instrumental contemporaneous offending was a significant predictor of multiple homicide. These results constitute exploratory evidence suggesting that multiple homicide has a greater likelihood of occurring when prisonization and concurrent instrumental criminal offending are present. The theoretical and practical implications of these findings are discussed. (Contains 2 tables.)