In 1999-2000, a measles epidemic occurred in The Netherlands, with 3292 reported cases; 94% of the affected patients had not been vaccinated. Only 1 patient had received 2 doses of vaccine. Three patients died, and 16% had complications. For the unvaccinated population, the incidence per 1000 inhabitants 15 months to 14 years old increased from 83 (95% confidence interval [CI], 53-113), in municipalities with vaccine coverage rates < or =90%, to 200 (95% CI, 153-247), in municipalities with coverage rates >95%; for the vaccinated population, the incidence increased from 0.2 (95% CI, 0.1-0.4) to 1.4 (95% CI, 0.9-1.9). Unvaccinated individuals were 224 times (95% CI, 148-460 times) more likely to acquire measles than were vaccinated individuals; the relative risk increased with decreasing vaccine coverage. Herd immunity outside unvaccinated clusters was high enough to prevent further transmission. More case patients came from the vaccine-accepting population living among unvaccinated clusters than from individuals who declined vaccination and who lived among the vaccine-accepting population.