Participants Participants were adults ages 18-75, referred by clinicians or responding to advertisements. After a preliminary telephone screening, eligibility was determined by clinical interview and confirmed by structured interview with trained raters using the Clinician-Administered PTSD Scale (CAPS) and the Structured Clinical Interview for DSM-IV Axis I Disorders -- Patient Edition. Participants had a principal Diagnostic and Statistical Manual of Mental Disorders, 4th Edition (DSM-IV) diagnosis of chronic PTSD of at least moderate severity (CAPS score ≥50), and English or Spanish fluency. Bilingual clinicians treated and assessed individuals with Spanish language preference. Exclusion criteria were significant suicidal ideation; lifetime psychotic disorder, bipolar disorder, organic mental disorder, or seizure disorder; alcohol or substance use disorder in the past 3 months; unstable medical illness; history of traumatic brain injury of greater than moderate severity; pregnancy or nursing; unwillingness to use contraception (for women of childbearing potential); prior nonresponse to sertraline or combined treatment, or intolerance of sertraline or mirtazapine); and psychotropic medication use during the prior 2 weeks (4 weeks for monoamine oxidase inhibitors or fluoxetine), except that zolpidem for insomnia was allowed up to three times per week during the week prior to randomization; psychotherapy initiated within 3 months before randomization. Concomitant psychotropic medications were not permitted during the study.