Measles outbreaks continue to occur in the United States and are mostly due to infections in returning travelers. To describe how providers assessed the measles immunity status of departing U.S. adult travelers seeking pretravel consultation and to assess reasons given for nonvaccination among those considered eligible to receive the measles, mumps, rubella (MMR) vaccine. Observational study in U.S. pretravel clinics. 24 sites associated with Global TravEpiNet (GTEN), a Centers for Disease Control and Prevention-funded consortium. Adults (born in or after 1957) attending pretravel consultations at GTEN sites (2009 to 2014). Structured questionnaire completed by traveler and provider during pretravel consultation. 40 810 adult travelers were included; providers considered 6612 (16%) to be eligible for MMR vaccine at the time of pretravel consultation. Of the MMR-eligible, 3477 (53%) were not vaccinated at the visit; of these, 1689 (48%) were not vaccinated because of traveler refusal, 966 (28%) because of provider decision, and 822 (24%) because of health systems barriers. Most MMR-eligible travelers who were not vaccinated were evaluated in the South (2262 travelers [65%]) or at nonacademic centers (1777 travelers [51%]). Nonvaccination due to traveler refusal was most frequent in the South (1432 travelers [63%]) and in nonacademic centers (1178 travelers [66%]). These estimates could underrepresent the opportunities for MMR vaccination because providers accepted verbal histories of disease and vaccination as evidence of immunity. Of U.S. adult travelers who presented for pretravel consultation at GTEN sites, 16% met criteria for MMR vaccination according to the provider's assessment, but fewer than half of these travelers were vaccinated. An increase in MMR vaccination of eligible U.S. adult travelers could reduce the likelihood of importation and transmission of measles virus. Centers for Disease Control and Prevention, National Institutes of Health, and the Steve and Deborah Gorlin MGH Research Scholars Award.