Using hierarchical logistic regression with a nationally representative sample of state prisoners ("n" = 12,504), we found inmates with dual severe psychiatric and substance abuse disorders to be at higher risk of being assaulted and to assault others in prison than nonmentally ill inmates. Dually disordered inmates may be "importing" characteristics that put them at more risk of involvement in assaults. Next, more than 50% of assault victims were themselves the perpetrators of assault, and significant percentages of inmates reported posttraumatic stress disorder (PTSD) diagnoses and physical and sexual victimizations. With other studies linking PTSD and being assaulted with revictimization and violence toward others, substance abuse, and poorer psychiatric outcomes, a study implication is providing inmates with effective trauma-relevant treatments. (Contains 2 tables and 6 notes.)