This study is a secondary analysis of data from the 1987 and 1992 National Health Interview Surveys. Analyses compared adults who do not have a usual source of care and those who identified usual sources of care in 1987 and 1992. Between these years, the estimated number of adult Americans without a usual source of care rose from 29.7 to 39.4 million. Adults were 0.75 times less likely to identify a physician's office and 1.8 times more likely to identify an outpatient clinic as that source of care in 1992 than they were in 1987. These changes were observed among Americans of all demographic and socioeconomic backgrounds. Increasing numbers of adult Americans without a usual source of care and shifts in care from physicians' offices to outpatient clinics may reflect deteriorating access to care. This may affect quality and costs of medical care, demanding continued surveillance of sources and access to care.