OBJECTIVES: Patients with insomnia are likely to have comorbidities that could affect treatment options. Hence the objective of this study was to examine the prevalence of comorbidities and their impact on the pharmacological treatment of insomnia in US primary care settings. METHODS: A retrospective data analysis of the National Ambulatory Medical Care Survey from 1995 to 2004 was performed. Patients aged = 18 years, who had a physician visit with a diagnosis of insomnia in US outpatient settings were included in this study. Office visits of patients with primary or secondary insomnia/ sleep complaints and resultant diagnoses were included in the analysis. Data were stratified according to patient characteristics, physician specialty, resulting diagnosis and medications prescribed. Multivariate logistic regression models were used to examine impact on prescribing pharmacotherapy for insomnia. RESULTS: A total of 5487 unweighted patient visits for insomnia were identified from the year 1995-2004, representing 107.4 million patients in the overall U.S. population. Official visits for insomnia were more common in females (60.4%), with an increasing prevalence in older patients. Approximately 41% of the patients with insomnia had a concomitant diagnosis of a mental comorbidity with higher prevalence of anxiety (15.6%) followed by episodic mood disorders (14.9%) and depression (7%). Patients with mental comorbidities were 35% less likely to receive pharmacological treatment for insomnia than those without mental comorbidities (OR: 0.65, 95% CI: 0.51-0.84). Subgroup analysis of type of mental comorbidity revealed that patients with comorbid anxiety were 42% less likely to receive pharmacological treatment for insomnia than those without anxiety (OR: 0.58, 95% CI: 0.45-0.73). CONCLUSION: Mental comorbidities such as episodic mood disorder, anxiety, and depression are prevalent in patients with insomnia and affect receipt of pharmacological therapy for insomnia. Health care professionals should consider the impact of mental comorbidities while treating patients with sleep difficulties.