Traditional group and staff model HMOs have contributed to public health investigations for decades. HMOs offer several advantages for this type of research because of their defined population and provider groups and the integrated nature of care delivery in these organizations. Traditional HMOs have also made investments in sophisticated data systems to support evidence-based care that is supported by high-quality clinical data available in automated information systems. This paper reviews why traditional HMOs are ideal places to conduct public health research and analyzes how recent market trends may threaten this role for managed care.