OBJECTIVES: While controlling for relevant and possibly confounding variables this study examined antidepressant treatment patterns for Florida Medicaid recipients with a diagnosis of MDD. The objective was to identify the types of antidepressant medication being prescribed including tricyclic antidepressants (TCAs), selective serotonin reuptake inhibitors (SSRIs), serotonin and nor-epinephrine reuptake inhibitor (SNRIs) (e.g.Venlafaxine and Duloxetine) and Bupropion. Dosage patterns and the factors that predict service use and costs among recipients of antidepressant therapy were analyzed. METHODS: The sample included 25,306 patients with a diagnosis of MDD who were enrolled in the Florida Medicaid Program during FY03-05. The research used three years of Medicaid claims data to compare patient demographics, diagnostic characteristics, antidepressant medication use, index dosage and service expenditure patterns 6 months prior to and one year after the index prescription event for antidepressant medication users. RESULTS: Among those persons diagnosed with MDD, the majority (84.5%) received SSRIs alone or in combination with other antidepressants, 29.2% received SNRIs alone or in combination with other antidepressants and 15.3% received TCAs alone or in combination with other antidepressants. A large majority (86.5%) were also receiving prescription pain medication. Of these, more than half were receiving prescription narcotics. Predictors of increased service use and cost following initiation of antidepressant treatment included older age, female sex, and pre-index prescription costs. Results showed that physical health care costs increased after switching to any antidepressant, however behavioral health costs tended to decrease for all treated patients overtime with the most dramatic decreases occurring in inpatient services. CONCLUSION: Current data on patterns, predictors and outcomes of antidepressant medication provided to patients with MDD are important to understand the care provided, promote optimal clinical practice and improve quality of care.