This study compared estimates of nurse practitioner (NP) supply in 12 states (statewide and rural vs. urban) derived from two sources: state license records and National Provider Identifier (NPI) data. Estimates of state NP supply from license data were found to be higher than NPI-derived estimates for most, but not all states. While data from both license and NPI sources can be useful for health workforce planning, the limitations of each source should be acknowledged and workforce comparisons should be limited to estimates derived from the same types of data.