The purpose of this study is to examine the causes and effects of State regulation that determines the extent of professional independence of advanced practice nurses (APNs). We analyze determinants of these regulations in panel data across States. We find that in States where APNs have acquired a substantial amount of professional independence, the earnings of APNs are substantially lower, and those of physicians' assistants are substantially higher, than in other States. These results are striking since physicians' assistants are in direct competition with APNs; the only real operational difference between these groups is that physicians' assistants are salaried employees who must work under the supervision of a physician. The implication is that physicians have responded to an increase in professional independence of APNs by hiring fewer APNs and more physicians' assistants.