2. 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. Different MPR thresholds (0.8,0.6,0.4 and 0.2) were used to determine non adherence. Associations between depressive symptoms, medication adherence and health care costs were assessed using ordinary least square multiple regressions.