AbstractHealth Maintenance Organizations (HMOs) have existed in the United States for at least 50 years. Recently their membership growth has increased markedly so that they currently care for millions of people in many locations in the United States. The growth of this form of medical care provides investigators with significant opportunities for research of all types. We will discuss some features of HMOs that facilitate research, concentrating on opportunities in the area of oncology. Our focus will be on not-for-profit HMOs, particularly ones in existence for a period of time sufficient to develop adequate numbers of members for study.