Volunteer-based clinics have existed in the United States for decades providing health care to the uninsured and underinsured. These free clinics all share a common dependence on volunteer staff and philanthropic provision of financial and non-financial resources. While several articles describe how these clinics operate or the role they play in the larger safety net, reports on large scale surveys are absent from the literature. This article describes a survey of volunteer-based clinics in seven Midwestern states, conducted in cooperation with the Robert Wood Johnson Foundation's Volunteers in Health Care resource center. The survey instrument was a four-page questionnaire that asked about 1998 clinic operations, the population served, the volunteers and staff providing care and services provided. Of the 106 clinics that responded, half described their target area as mixed, a quarter as urban, about a sixth as rural, and about a twelfth as suburban. Almost two-thirds of the clinics reported annual budgets of less than $200,000. The primary patient population of these clinics tended to be low-income, uninsured individuals, though some clinics also accepted the underinsured as well. Altogether, these clinics provided care to over 200,000 patients. The authors estimate that there may be as many as 1,000 free clinics providing health services nationwide. While this is a rapidly growing sector, the authors assert that free clinics are not currently an adequate means of addressing the health care needs of the increasing number of uninsured and underinsured populations in the United States. Future research should focus on the racial/ethnic mix of patients served by volunteer-based clinics and on providing a more conclusive count of clinics nationwide and the patients they serve.