Mumps, an acute vaccine-preventable viral illness transmitted by respiratory droplets and saliva, has an incubation period most commonly of 16-18 days. The classic clinical presentation of mumps is parotitis, which can be preceded by several days of nonspecific prodromal symptoms; however, mumps also can be asymptomatic, especially in young children. Mumps transmission can occur from persons with subclinical or clinical infections and during the prodromal or symptomatic phases of illness. In 2006, during a mumps resurgence in the United States, the latest national recommendations from CDC and the American Academy of Pediatrics (AAP) stipulated that persons with mumps be maintained in isolation with standard precautions and droplet precautions for 9 days after onset of parotitis. However, the existence of conflicting guidance (i.e., that the infectious period of mumps extended through the fourth day after parotitis onset) led to confusion regarding the appropriate length of isolation. In addition, during the 2006 resurgence, compliance with recommendations for isolation in university settings was substantially lower for 9 days (65%) compared with 4-5 days (86%). In 2007, after a review of the evidence supporting the 9-day isolation guidance by AAP and CDC, AAP changed its isolation guidance for health-care workers in ambulatory settings from 9 days to 5 days. In February 2008, after review of data on mumps in health-care settings, mumps viral load, and mumps virus isolation, the Healthcare Infection Control Practices Advisory Committee (HICPAC) approved changes in its recommendations related to mumps in in-patient settings. As a result, CDC, AAP, and HICPAC all now recommend a 5-day period after onset of parotitis, both for isolation of persons with mumps in either community or health-care settings and for use of standard precautions and droplet precautions. This report summarizes the scientific basis for these changes in mumps isolation guidance.