With more than 5 million patient visits annually, certified nurse-midwives (CNMs) substantially contribute to women's health care in the United States. The objective of this study was to describe ambulatory visits and practices of CNMs, and compare them with those of obstetrician-gynecologists (OB/GYNs). Sources of population-based data used to compare characteristics of provider visits were three national surveys of CNMs and two National Ambulatory Medical Care Surveys of physicians. When a subset of 4,305 visits to CNMs in 1991 and 1992 were compared to 5,473 visits to OB/GYNs in similar office-based ambulatory care settings in 1989 and 1990, it was found that a larger proportion of CNM visits were made by women who were publicly insured and below age 25. The majority of visits to CNMs were for maternity care; the majority of visits to OB/GYNs were for gynecologic and/or family planning concerns. Face-to-face visit time was longer for CNMs, and involved more client education or counseling. This population-based comparison suggests that CNMs and OB/GYNs provide ambulatory care for women with diverse demographic characteristics and differing clinical service needs. Enhancing collaborative practice could improve health care access for women, which would be especially beneficial for those who are underserved and vulnerable.