In August 2008, CDC and the Puerto Rico Department of Health conducted a serosurvey of patients who had discontinued rabies postexposure prophylaxis (PEP) prior to completing a schedule of five vaccine doses. The objective was to determine whether further vaccination of these patients was needed based on serum rabies neutralizing antibody levels. Eighteen patients consented to serology using the rapid fluorescent focus inhibition test. The World Health Organization's cutoff value of 0.5. IU/mL was used as the basis for recommending PEP continuance, while complete virus neutralization at the 1:5 dilution indicated seroconversion per current Advisory Committee for Immunization Practices recommendations. Serum samples were collected a median of 147 days (range 24-215) after receipt of the last vaccine dose. Ten patients were recommended for PEP continuance for titers below 0.5. IU/mL; however, of 11 patients, 33% of 2-dose, 100% of 3-dose, and 100% of 4-dose patients exhibited seroconversion. These findings corroborate previous studies that suggest a less than five-dose rabies vaccine regimen elicits adequate immunogenicity against rabies. © 2010.