Since 1963, 11 cases have been reported in which both the acardiac twin and the "normal" co-twin were studied cytogenetically. Aneuploidy or polyploidy was clearly identified in the acardiac twin in 7 cases and in the co-twin in 1 case. We report on 2 additional twin pairs in which aneuploidy was associated with acephalus-acardia. In both cases the "normal" co-twin had a Klinefelter (47,XXY) karyotype. Chromosome analysis in the 2 acardiac twins documented a 47,XXY constitution in one and 94,XXXXYY anomaly in the other. One of the "normal" co-twins also had the VATER association. Given these data we would recommend chromosome analysis of both members of a twin pair when one has acephalus-acardia.