A cost-effectiveness analysis was conducted to determine the relative efficiency of nine primary health care projects serving low income persons in urban and rural areas of Texas. The projects were created as part of a government program to provide services in medically underserved areas of the state by contracting with local public and private providers. While they vary widely in terms of sponsoring organizations, service strategy, and scope of services, the projects share a similar goal of providing an array of basic medical and preventive care services to the poor. This study compares the average cost per encounter for diagnosis/treatment, emergency, family planning, and preventive health screening services provided by the various projects during the first year of operation. After adjusting for differences in input prices and patient volume, the cost differences are examined and related to organizational model and personnel mix. Those projects with the highest ratio of nonphysician to physician practitioners proved more cost-effective, as did those projectsfollowing the public clinic model of service delivery.