Weight gain has often been associated with olanzapine treatment, yet little is known about the influence of patient age or cumulative dose on olanzapine-associated weight gain. The first 118 participants in the National Institutes of Mental Health Study of the Pharmacotherapy of Psychotic Depression randomized clinical trial (ClinicalTrials.gov Registration NCT00056472) completing at least 4 weeks of treatment with olanzapine were analyzed to determine the relationship between weight gain, age, and cumulative olanzapine dose. Younger (age 18-59 years) and older (age 60+ years) participants received open-label olanzapine and either sertraline or placebo for up to 12 weeks. Linear mixed effect regression modeling was used to determine the effects of age and cumulative olanzapine dose on weight gain, controlling for potential confounders. Age was observed to have a significant negative association with weight gain (P=0.01), even after controlling for differences in cumulative dose and baseline body mass index. Each 10-year increase in age was associated with a decrease in mean weight gain over 12 weeks of approximately 0.6 kg (95% confidence interval: 0.14-1.05 kg). Cumulative olanzapine dose was also significantly associated with weight gain (P<0.0001). Approximately 60% of completers of the 12-week trial experienced clinically significant weight gain (> or =7% of baseline weight).