Background: Interpersonal firearm violence is a serious public health problem. Individuals with a firearm injury are at particularly high risk of subsequent firearm-related injury and death, but risk factors for reinjury or death among people with an initial firearm assault injury are not well understood. This study aims to identify these risk factors and quantify the overall and transition-specific hazards of subsequent assaultive injuries or death among people in this population. Methods: In this retrospective cohort study, we use California’s hospital discharge, emergency department, and mortality records, 2005-2013, to identify and follow individuals with an initial firearm assault injury. We used multi-state models to explicitly and simultaneously model transitions from one injury to the next or from an injury to death, accounting for event history and other covariates. These models allow the hazards to change with each subsequent event and account for competing risks. Results: Each firearm assault injury increased the probability of experiencing a subsequent fatal or nonfatal firearm assault in a dose-response manner. Relative to women, men had an increased hazard of reinjury (hazard ratio [HR]: 2.92, 95% CI: 2.11, 4.05) and death (HR: 4.58, 95% CI: 2.36, 8.90) from a firearm assault; and relative to whites, blacks had increased hazards (reinjury HR: 2.41, 95% CI: 1.86, 3.14; death HR: 2.17, 955 CI: 1.41, 3.34). Conclusions: By identifying characteristics of patients at especially high risk of assaultive firearm reinjury or death, this study can inform clinical decision making and the targeting of preventive interventions and policies nationwide.