OBJECTIVES: To evaluate the impact of Generalized Anxiety Disorder (GAD) on diagnosed comorbidities, medication use and treatment costs. METHODS: Claims were drawn from PharMetrics Integrated Outcomes Database for 12-month prior and post the first GAD (ICD9-CM: 30002) diagnosis between 1/2003 and 6/2004 (the diagnosis date as the index date). No GAD diagnosis 12-month prior the index date, 24-month continuous insurance eligibility and aged 18-64 were required. Changes in diagnoses of comorbidities, medication use patterns, and treatment costs between the year before and after the index date were examined. Comparison among subgroups of GAD patients with comorbid depression and pain was also investigated. Wilcoxon Signed Rank test and McNemar's test were used to examine pre-post differences for continuous and categorical variables, respectively. RESULTS: A total of 240,041 patients were included in this study. The mean age was 41.7 years old and 67% were female. After diagnosis of GAD, a significantly higher percent of patients were diagnosed with depression (44.4% vs. 30.9%, P<.001), dyslipidemia (24.0% vs.19.9%, p<.001), and diabetes (6.1% vs. 5.3%, p<.001) than before GAD diagnosis. The use of antidepressants increased from 42.3% to 56.8% (p<.001). Compared to the year prior to GAD diagnosis, total annual costs increased by $2,034 (p<.001) driven mainly by increases of inpatient and outpatient costs ($285 and $773, p<.001, GAD only and increased merely $306 for GAD patients with depression while GAD patients with pain and those with both pain and depression increased by $2,253 and $4,665 (both p<.001), respectively. CONCLUSION: Diagnosis of GAD had significant impact on comorbidities, medication use and treatment costs. Furthermore, comorbid pain and depression had substantial extra burden on GAD patients as compared with those had GAD only. Recognizing these comorbidities is important in the treatment of patients with GAD.