Because of increased measles incidence in the United States during 1989 and 1990 and the recent finding of genomic differences between vaccine virus and contemporary wild measles viruses, we conducted a study to determine whether the current measles vaccine had become less effective. Household secondary attack rates for 203 California children ages 1 to 5 years were 4.2 and 77.8% for vaccinated and unvaccinated children, respectively, and the vaccine efficacy was 95% (95% confidence interval: 89%, 97%). The protective efficacy for postexposure vaccination and use of IG were both low, 4% (95% confidence interval: less than 0, 36%) and 8% (95% confidence interval: less than 0, 59%), respectively. The measles vaccine efficacy found in this study is similar to those obtained in previous years and indicates that the measles epidemic of 1989 to 1990 occurred despite high vaccine effectiveness.