A 46-year-old woman who has three children, whose red blood cells exhibit a dual population of groups A and B, was investigated. Her lymphocytes had an XX/XY karyotype and Y chromosome markers were detected from buccal cells and hair root cells in DNA analysis. Three bands were detected in D1S80 VNTR locus (MCT118), two bands from father and one from mother. Her ABO genotype should be genetically AO, however, the PCR-RFLP method revealed a pattern of BO genotype. From these findings, it is concluded that she is a so-called dispermic chimera which is the result of the fertilization by two sperms and that DNA analysis is useful for distinguishing between twin and dispermic chimerism.