Birth registration is not only a fundamental human right, but also a requirement for obtaining additional documents, proving legal identity, and accessing a number of government benefits. Yet, little is known about the effects of birth under-registration on access to health care. Using data from the Dominican Republic, this paper is the first to shed light on the causal impact of the lack of birth registration on childhood immunization, one of the key components of public services in many developing countries. Controlling for potential endogeneity and standard socioeconomic determinants of immunization, this paper finds that children between 0 and 59 months of age that do not have birth certificates are behind by nearly one vaccine (out of a total of nine) compared to those that have birth certificates. The results are robust to several robustness tests and threats to the exclusion restriction of the instrumental variables. Birth underregistration specifically reduces the probability of vaccination against polio, diphtheria, pertussis, and tetanus-once leading causes of child morbidity and infant mortality. In addition, untimely vaccination costs governments billions per year in treatment and rehabilitation.