Because the atypical antipsychotic olanzapine may be efficacious in treating post-traumatic stress disorder (PTSD) symptoms, we conducted a 10-week, double-blind, placebo-controlled evaluation in which 15 patients were randomized 2: 1 to either olanzapine or placebo. The initial dosage was 5 mg/day and was titrated to a maximum of 20 mg/day. Eleven patients completed the study. Patients in both groups showed improvement in PTSD symptoms, but no between-group differences in treatment response were observed and a high placebo response rate was found. Both treatments were tolerated well, although the olanzapine treatment group had more weight gain. Olanzapine fared no better than placebo in this preliminary study in the treatment of PTSD. The lack of difference between olanzapine and placebo may in part be due to olanzapine's not being effective in PTSD or, alternatively, a small sample size, a high placebo response in certain forms of PTSD and the chronicity of PTSD symptoms in some patients.