The current study examined the longitudinal effects of clinical and treatment utilization factors on aggressive behavior among 376 help-seeking U.S. veterans recently diagnosed with posttraumatic stress disorder (PTSD) who were followed for 5–12 months. Participants were sampled from 4 strata: male Iraq/Afghanistan veterans, female Iraq/Afghanistan veterans, male prior-era veterans, and female prior-era veterans. Hierarchical regression analyses indicated that changes in PTSD severity were significantly associated with changes in aggressive behavior among veterans who reported any aggression at baseline (β = .15). Changes in days of alcohol intoxication also were positively associated with changes in aggressive behavior (β = .16). Participants with both a benzodiazepine prescription and any baseline aggression were significantly more likely to increase in aggressive behavior over time (β = .14). Contrary to our hypotheses, reductions in aggressive behavior were not related to the number of outpatient mental health visits or to first-line recommended psychotropic medications. Results inform assessment and clinical research on changes in aggressive behavior among veterans with PTSD.