Study objective: Two widely used formulas for calculating the number of practicing emergency physicians (EPs) are based on the total number of US emergency departments and patient visits in this study we hypothesized that the number of physicians now working in EDs is significantly greater than the estimates yielded by these formulas. Therefore we attempted to determine the accuracy of these methods for predicting the true number of practicing EPs. We also examined the training, board certification, and distribution of EPs. Methods: The EDs of all hospitals listed by the Missouri Hospital Association (MHA), excluding children's and psychiatric hospitals, were surveyed over a 9-month period in 1994 with regard to the number and board status of all physicians practicing in their EDs and the numbers of full-time equivalents (FTEs) required for adequate staffing. These numbers were compared with 1994 estimates for Missouri based on two common methods of calculation. Results: Of 134 hospitals with EDs, 118 (88%) completed our survey. These EDs employed 458 full-time EPs and 690 part-time EPs, with 41% and 7% board certified in emergency medicine, respectively. Board-certified emergency physicians were concentrated in large cities and at university hospitals and were sparsely represented in rural areas. Adequate staffing of these EDs required 677 FTEs, compared with estimates of 358 (formula A) and 555 (formula B). Previously published formulas underestimate the need for EPs in our state by 47% (formula A) or 18% (formula B). Conclusion: Current staffing estimates regarding EPs working in Missouri greatly underestimate actual staffing needs. Board certified EPs are in severe shortage and are unequally distributed in Missouri. Extrapolated nationally, these estimates may negatively affect funding and available residency positions for emergency medicine.