OBJECTIVES: Compare comorbidity profiles and direct costs in older patients with depression treated or untreated with antidepressants and matched controls without depression. METHODS: Administrative claims from a multi-specialty medical group practice in central Massachusetts were used to identify 1777 patients age ≥65 years as of January 1, 2006 with continuous medical and pharmacy coverage in 2005 (baseline) and 2006 (study period) and ≥1 depression diagnosis in 2005. Patients with ≥1 antidepressant claim in 2006 were classified as "treated" and those without an antidepressant as "untreated." Depressed patients were randomly matched to controls without depression on age and gender. Baseline comorbidity profiles were compared using Chi-square tests. Wilcoxon rank-sum tests and generalized linear models were used to compare study period direct (medical and pharmacy) costs defined as third party payments to providers. Depression-related costs were identified using claims with depression diagnosis and antidepressant prescriptions. RESULTS: A total of 1334 (75.1%) of depressed patients were treated with antidepressants during the study period and 443 patients (24.9%) were not treated. Depressed patients were on average 77 years old and 74.8% were women. Depressed patients had higher rates of mental health and physical comorbidities than controls. Treated depressed patients had higher rates of mental health comorbidities than the untreated but few differences in rates of physical comorbidities. During the study period average annual direct costs were $14,362 among treated depressed patients, $8,928 among the untreated, and $6,961 among controls (P<0.01 for all comparisons). Risk-adjusted direct costs were significantly different between the treated and untreated depressed. Average depression-related costs were $1,582 among the treated and $133 among the untreated (P<0.01). CONCLUSIONS: Older depressed patients (both treated and untreated) had more comorbidities and higher costs compared to controls. Costs and rates of mental health comorbidities were higher among the treated depressed compared with the untreated.