To evaluate the recent decision of the Advisory Committee on Immunization Practice to increase the recommended age for initial measles vaccination from 12 to 15 months, we carried out a case control study of vaccine failure in a recent measles epidemic. Compared to children vaccinated at ages 15 months or older, we found an increased risk of vaccine failure among those vaccinated at 12 to 14 months (relative risk = 19.2, 95% confidence interval = 4.6 to 80.1). In order to sort out the influence of age at vaccination from elapsed time since vaccination, we subjected the data to discriminant analysis. Age at vaccination subsumed all of the effect of duration of time since vaccination. Thus, we find no evidence of waning immunity over time.