OBJECTIVES: The effect of insomnia along with comorbidities associated with aging is a serious concern for potential health behaviors in the elderly (65 years and older) population. We examined predictors of medication adherence in insomnia in Medicare-HMO enrolled elderly patients, controlling for the effects of confounding variables. METHODS: This was a retrospective, longitudinal cohort study which included elderly patients (65 and older) enrolled continuously for 1-5 years in the Medicare HMO. Medication possession ratio was used to estimate the adherence in insomnia medication. Association between various predictors of adherence in insomnia was assessed using ordinary least square multiple regressions, accounting for potential confounders like general health status, IADL and severity of comorbidities. RESULTS: A total of 1338 patients suffering depressive symtomatology with a primary diagnosis of insomnia were included in the study. Presence of depressive symptomatology in this population (prevalent in 60% of the study population) was associated with a 33% decrease in medication possession ratios compared to patients without depressive symptoms. CONCLUSIONS: We found strong associations between depressive symtomatology and  medication adherence in elderly patients with insomnia. Disease and risk management programs in managed care settings should be used to optimize the medication adherence in the elderly.