Physician mergers involve 38% of doctors, substantial health system participation, and frequent serial acquisition.

We describe the incidence of mergers between US physician firms. We considered the role of health systems in physician merger activity and the extent to which individual firms engage in multiple acquisitions, or "roll-up" behavior. We used a unique collection of subpoenaed data from 6 insurers in 15 states, which account for approximately 80% of commercially insured members in those states between 2015 and 2020. Over 6 years, we observed 2019 mergers between physician firms that spanned from small single-specialty practices to large groups affiliated with health systems. Approximately 20% of the roughly 15 000 firms were involved in a merger. Of the nearly 400 000 doctors in our sample, 38% were employed by firms involved in a merger, although only 3.8% of the doctors were part of an acquired group. Sixty-three firms made more than 5 acquisitions, and 3 made more than 20 acquisitions. Health care systems, responsible for approximately 40% of mergers, frequently engaged in serial acquisitions. Acquired physician firms are typically small (<10 doctors), and much smaller than the acquiring firm. We document that many physician mergers occurred in 15 states over 5 years. A significant fraction of the physician mergers we identified, especially those involving health systems, exhibited characteristics that could indicate the potential for competitive harm. Our results suggest that researchers should examine the degree to which these competitively suspect mergers have affected competition.