Sero-positivity rates of the rubella virus among pregnant women vary widely throughout the world. In Tanzania, rubella vaccination is not included in the national immunization schedule and there is therefore no antenatal screening for this viral disease. So far, there are no reports on the sero-prevalence of rubella among pregnant women in Tanzania. As a result, this study was undertaken to establish the sero-positivity rate of rubella and rubella risk factors among pregnant women attending antenatal care clinics in Mwanza, Tanzania.