BACKGROUND Most proposals to increase access to primary care in the United States emphasize increasing the proportion of generalist physicians. Another approach is to increase the number of physician assistants, nurse practitioners, and certified nurse-midwives. METHODS We analyzed variations in the regulation of nurse practitioners, physician assistants, and certified nurse-midwives in all 50 states and the District of Columbia. Using a 100-point scoring system, we assigned numerical values to specific characteristics of the practice environment in each state for each group of practitioners, awarding a maximum of 20 points for legal status, 40 points for reimbursement for services, and 40 points for the authority to write prescriptions. We calculated coefficients for the correlation of summary measures of these values within states with estimates of the supply of practitioners per 100,000 population. RESULTS There was wide variation among states in both practice-environment scores and practitioner-to-population ratios for all three groups of practitioners. We found positive correlations within states between the supply of physician assistants, nurse practitioners, and certified nurse-midwives and the practice-environment score for the state (Spearman rank-correlation coefficients, 0.63 [P < 0.001], 0.41 [P = 0.003], and 0.51 [P < 0.001], respectively). Positive associations were also found in the states between the supply of generalist physicians and the supply of physician assistants (r = 0.54, P < 0.001) and nurse practitioners (r = 0.35, P = 0.014). Nevertheless, in the 17 states with the greatest shortages of primary care physicians, favorable practice-environment scores were still associated with higher practitioner-to-population ratios for physician assistants (r = 0.68, P = 0.003), nurse practitioners (r = 0.54, P = 0.026), and certified nurse-midwives (r = 0.42, P = 0.09). CONCLUSIONS State regulation of physician assistants, nurse practitioners, and certified nurse-midwives varies widely. Favorable practice environments are strongly associated with a larger supply of these practitioners.