The completeness of rabies postexposure prophylaxis (PEP) reporting was evaluated in King County, Washington State. Information on rabies immune globulin prescriptions was obtained from hospital pharmacies associated with emergency departments in King County from 2003 to June 2006. Rabies immune globulin is given at the initiation of rabies PEP which is usually started at emergency departments. Because pharmacies are not regular sources of rabies PEP reporting, we compared pharmacy cases with cases reported via routine passive surveillance methods. A capture-recapture method was used to calculate the estimated number of unreported cases from all sources. Reporting completeness was calculated by dividing the number of cases reported via routine surveillance with the sum of reported and estimated unreported cases. Seventy-one unreported rabies PEP cases were identified by comparing previously reported cases with pharmacy cases. A total of 128 cases were estimated to have been missed by the surveillance system. Overall reporting completeness was 62 percent increasing to almost 80 percent in 2005 and 2006. Our findings illustrate the importance of evaluating surveillance systems and suggest that it may be useful to institute active rabies PEP surveillance with emergency departments in addition to continuing educating healthcare providers and facilities about reporting.