We report the first case of hexasomy 12p mosaicism due to 2 copies of an apparent i(12p) [46,XX/48,XX, +i(12p), +i(12p)]. In every cell that contained the i(12p), 2 copies of the marker were found. The hexasomy was found in amniocytes (16%) and skin fibroblasts (95%) but not in peripheral blood lymphocytes. The chromosomal origin of the marker was confirmed with the use of in situ hybridization of alpha-satellite specific for the centromere of chromosome 12. The present case was diagnosed following chromosome analysis for anomalies on ultrasound. The hexasomy 12p patient showed striking phenotypic similarities with severely affected tetrasomy 12p cases and died shortly after birth. We propose that the more severe presentation of this case is due to the 4 extra copies of 12p.