Objective: Patients with insomnia are likely to have other co-morbidities that could affect pharmacotherapeutic choices. This study examined the prevalence and impact of co-morbidities on the pharmacological treatment of insomnia. Study design: A retrospective data analysis of the National Ambulatory Medical Care Survey from 1995 to 2004, comprising patients with a diagnosis of insomnia, was conducted. Multivariate logistic regression models were used to predict the impact of co-morbidities on pharmacotherapy for insomnia. Results: A total of 5,487 unweighted patient visits with insomnia were identified, representing 161.4 million patients in the US. Approximately 38% of these patients had at least one co-morbid condition. Patients with mental co-morbidities, especially anxiety, had decreased likelihood of receiving pharmacotherapy for insomnia. Conclusions: Mental co-morbidities such as episodic mood disorder, anxiety and depression are prevalent in patients with insomnia. However, many co-morbid patients do not receive pharmacological therapy specific for insomnia.