In April 1989, New York became the first State in the United States to adopt a two-dose schedule for routine measles immunization. Although a two-dose schedule had been under discussion for the previous 10 years, this policy change was finally prompted in New York State by widespread measles outbreaks in 1989 among college and high school students who had been appropriately vaccinated with a single dose of measles vaccine. These outbreaks affected 21 college and secondary school campuses with 91 cases of measles and led to the administration of 53,093 doses of vaccine at a cost in excess of $859,000 for vaccine alone. In addition, there were major disruptions of intercollegiate athletic and scholastic events and physician and public confusion over the different recommendations for "outbreak" versus "routine" measles immunization. In response, the New York State Department of Health adopted a policy of two doses of measles vaccine required for entrance into kindergarten and college beginning in the fall of 1990. This report describes the data and process that were used in reaching this policy decision.