Although monochorionic (MC) dizygotic twins (DZT) are extremely rare in natural pregnancy, six pairs of such twins have successively been reported in a recent short period. All six cases of MC DZT were the products of pregnancy by assisted reproductive technology (ART). In this overview, we summarize these six cases and discuss possible mechanisms of this twinning and clinical implications of confined blood cell chimerism (CBC). The placental MC membrane was diagnosed ultrasonographically in all cases and pathologically in four. The presence of CBC was confirmed in four cases by haplotyping at polymorphic marker loci in peripheral blood leukocytes, karyotyping of lymphocytes and skin fibroblasts, and/or ABO blood group typing. As CBC is attributable to placental vessel anastomosis between DZT, it may become a risk factor for twin-twin transfusion syndrome (TTTS), mortality, and for other complications in twins. MC DZT may produce psychological trauma, especially in a girl/woman when she grows up and is known to be chimeric for a male karyotype and vice versa, although genital organs are generally normal--unlike freemartin in cattle. In addition, CBC in twins may mislead physicians when genotyping for a disease-susceptibility test is performed in medical practice in the near future. Blood group chimera may also cause confusion if a blood transfusion is necessary. Therefore, sufficient informed consent prior to ART and genetic counseling before/after birth are absolutely necessary for improved quality of life. It is most likely that all six cases are the consequence of fusion between two outer cell masses from two zygotes. The ART used in the six MC DZT included in vitro fertilization-embryonic transfer (IVF-ET) into the uterus, FSH-induced superovulation followed by intrauterine insemination, and/or intracytoplasmic sperm injection (ICSI). The use of an ovulation-inducing agent and implantation of several fertilized eggs at close sites are probably the events common among these cases. Assisted hatching, simultaneous ET, the use of eggs that have developed to the blastcyst stage, and cell culture procedures that lead to changes of the nature of cell surface, all may increase the chance of a cell fusion. This "chance hypothesis" can simply explain why MC DZT are very rare in natural pregnancy. Large-scale research on the prevalence of ART-associated MC DZT and long-term follow-up of the twins are essential.