The research reported here examined the factors which affected the decision to remain with either Blue Cross of Washington and Alaska or Group Health Cooperative of Puget Sound, or to change to an independent practice association (IPA) in which the primary care physicians control all care. The natural setting allowed examination of the characteristics of families with experience in structurally different plans; a decision not influenced by premium differentials; the importance of the role of usual provider; and a family-based decision using multivariate techniques. An expected utility model implied that factors affecting preferences included future need for medical care; access to care; financial resources to meet the need for care; and previous level of experience with plan and provider. Analysis of interview and medical record abstract data from 1,497 families revealed the importance of maintaining a satisfactory relationship with the usual sources of care in the decision to change plans. Adverse selection into the new IPA as measured by health status and previous utilization of medical services was not noted.