Abstract Studies concerning nine pregnant women inadvertently inoculated with live rubella virus vaccine confirmed that the attenuated virus can produce chronic placental infection. None of the women had been tested for rubella susceptibility before vaccination; however, serologic testing of later serum specimens indicated seroconversion of one. This subject and one other reported symptoms consistent with vaccine-virus infection. In both women the attenuated virus was isolated from conceptuses collected 69 and 28 days respectively after immunization. Attempts at virus isolation were negative on specimens from the remaining six women. Histologic changes in placenta or decidua consistent with rubella infection were detected in the two virus-positive cases and in one negative case. These data underscore the need for caution in vaccinating postpubertal women.