Few studies have examined what happened to child survival when DTP and OPV were introduced in low-income countries. These vaccines were introduced in 1981 in an urban community in Guinea-Bissau from 3 months of age in connection with 3-monthly weighing sessions. Children were therefore allocated by birthday to receive vaccines early or late between 3 and 5 months of age. In this natural experiment vaccinated children had 5-fold higher mortality than not-yet-DTP-vaccinated children. DTP-only vaccinations were associated with higher mortality than DTP + OPV vaccinations. Hence, DTP may be associated with a negative effect on child survival.