Background: There has been considerable focus on the uninsured from national and state levels. There are also many Americans who have health insurance but are unable to afford their recommended care and are considered underinsured. This purpose of this study was to determine the prevalence of underinsurance among patients seen in primary care clinics.Methods: Patients in 37 primary care practices in 3 practice-based research networks completed a survey to elicit the prevalence of underinsurance among those who had insurance for a full 12 months, including private insurance, Medicare, and Medicaid. Being underinsured was based on patients reporting the delay or omission of recommended care because of their inability to afford it.Results: Of those with insurance for a full year, 36.3% were underinsured. Of those who were underinsured, 50.2% felt that their health suffered because they could not afford recommended care, a rate similar among those who were uninsured.Conclusions: When evaluating underinsurance in primary care offices, using an experiential definition based on self-reports of patients about their inability to pay for recommended health care despite having insurance, the prevalence is quite high. It is important for the primary care physician to understand that a substantial percentage of their patients may not follow through with their recommendations because of cost, despite having insurance. This also has significant implications when considering health care reform, particularly considering that these patients reported that their health suffered at a rate equal to that of the uninsured.