Although injections administered during the incubation period of wild poliovirus infection have been associated with an increased risk of paralytic poliomyelitis, quantitative estimates of the risk have not been established. During a poliomyelitis outbreak investigation in Oman, vaccination records were reviewed for 70 children aged 5-24 months with poliomyelitis and from 692 matched control children. A significantly higher proportion of cases received a DTP (diphtheria and tetanus toxoids and pertussis vaccine) injection within 30 days before paralysis onset than did controls (42.9% vs. 28.3%; odds ratio, 2.4; 95% confidence interval, 1.3-4.2). The proportion of poliomyelitis cases that may have been provoked by DTP injections was 35% for children 5-11 months old. This study confirms that injections are an important cause of provocative poliomyelitis. Although the benefits of DTP vaccination should outweigh the risk of subsequent paralysis, these data stress the importance of avoiding unnecessary injections during outbreaks of wild poliovirus infection.