Objective: The aim of this study was to highlight the outcome of complete hyda tidiform mole (CHM) coexisting with a live co-twin. Methods: We investigated fo ur cases of such pregnancy by ultrasound, pathological, cytogenetic, and molecul ar techniques. Information on clinical follow-up and outcome was also available . Results: All four pregnancies were spontaneous: two ended with the delivery of a live-born baby, while the other two were terminated because of signs of seri ous maternal pathology or intrauterine fetal death. The criteria for carrying on with the pregnancy are reviewed. The immediate outcome depends on the maternal criteria of serious pathology and on the likelihood of intrauterine fetal death. The risk of persistent trophoblastic disease (PTD) is the same as in the case o f a singleton complete mole and also seems to be correlated with the zygosity me chanism identified by molecular analysis. Conclusion: Hydatiform mole with a liv e co-twin fetus is a rare obstetric occurrence. In the case of a normal fetal k aryotype, it is justifiable to await developments in the absence of maternal com plications. However, treatment criteria still need improvement and diligent mate rnal follow-up is always warranted in the postpartum period. Objective: The aim of this study was to highlight the outcome of complete hyda tidiform mole (CHM) coexisting with a live co-twin. Methods: We investigated fo ur cases of such pregnancy by ultrasound, pathological, cytogenetic, and molecul ar techniques. Information on clinical follow-up and outcome was also available. Results: All four pregnancies were spontaneous: two ended with the delivery of a live-born baby, while the other two were terminated because of signs of seri ous maternal pathology or intrauterine fetal death The criteria for carrying on with the pregnancy are reviewed. The immediate outcome depends on the maternal criteria of serious pathology and on the likelihood of intrauterine fetal death. The risk of persistent trophoblastic disease (PTD) is the same as in the case of a singleton complete mole and also seems to be correlated with the zygosity me chanism identified by molecular analysis. Conclusion: Hydatiform mole with a liv e co-twin fetus is a rare obstetric o In the case of a normal fetal k aryotype, it is justifiable to await developments in the absence of maternal com plications. However, treatment criteria still need improvement and diligent mate rnal follow-up is always warranted in the postpartum period.