OBJECTIVE: Posttraumatic stress disorder (PTSD), particularly in combat veterans with chronic illness, is often refractory to standard pharmacological interventions. There is a need to test adjunctive treatments to boost response. METHOD: Subjects were 19 patients with PTSD who were minimally responsive to 12 weeks of treatment with a selective serotonin reuptake inhibitor (SSRI) at maximum tolerated dose. Outcomes were compared among subjects whose treatment was augmented with 8 weeks of double-blind olanzapine or placebo administration. RESULTS: Olanzapine augmentation was associated with statistically significantly greater reduction than placebo in specific measures of posttraumatic stress, depressive, and sleep disorder symptoms. Clinician-rated global response rates did not, however, significantly differ between groups. CONCLUSIONS: This is most likely the first double-blind, placebo-controlled study of an adjunct to SSRIs for PTSD. Despite the small group size, the findings suggest a role for olanzapine or other atypical antipsychotics in treating SSRI-resistant PTSD. Sleep symptoms may especially benefit.