Federal policies and state legislation in the United States encourage the use of physician assistants (PAs) and nurse practitioners (NPs) in primary care, although the nature of their work has not been fully analyzed. This article analyzes primary care physician office-encounter data from the 1995-1999 National Ambulatory Medical Care Surveys. About one-quarter of primary care office-based physicians used PAs and/or NPs for an average of 11% of visits. The mean age of patients seen by physicians was greater than that for PAs or NPs. NPs provided counselling/education during a higher proportion of visits than did PAs or physicians. Overall, this study suggests that PAs and NPs are providing primary care in a way that is similar to physician care.