Context: A 3-year pilot program to expand the role of nurse practitioners (NPs) in the Washington State workers' compensation system was implemented in 2004 (SHB 1691), amid concern about disparities in access to health care for injured workers in rural areas. SHB 1691 authorized NPs to independently perform most functions of an attending physician. Purpose: The aims of this study were to (1) describe the contribution by NPs to Washington's workers' compensation provider workforce, (2) evaluate change in provider availability attributable to SHB 1691, and (3) evaluate the effect of SHB 1691 on timely accident report filing. Methods: Administrative data were used to evaluate this natural experiment, using a pre-post design with primary care physicians (PCPs) as a nonequivalent comparison group. Findings: NPs served injured workers with characteristics similar to those served by PCPs, but 22.0% of NPs were rural, compared with 17.3% of PCPs. Of claimants with NPs as their attending provider, 53.3% were injured in a rural county, compared with 24.7% for those with PCP attending providers. The number of NPs participating in the workers' compensation system rose after SHB 1691 implementation, more so in rural areas. SHB 1691 implementation was associated with a 16 percentage point improvement in timely accident report filing by NPs in both rural and urban areas. Conclusions: Authorizing NPs to function as attending providers for injured workers may improve provider availability (especially in rural areas) and timely accident report filing, which in turn may improve worker outcomes and system costs.