We report a patient with trisomy X and a supernumerary marker chromosome. The marker chromosome was characterized by comparative genomic hybridization and shown to be derived from chromosome 14, resulting in trisomy for 14pter → q21. The karyotype was thus redefined as 48,XXX,+mar.rev ish enh(14pterq21). The patient presented with facial dysmorphism and a high‐pitched cry, exhibited severe developmental delay, and developed an aggressive ovarian immature teratoma. In this paper, we also review reports of 11 other patients with constitutional trisomy of the same chromosomal region. Previous studies have identified somatic gains of chromosome 14 in ovarian germ cell tumors. We propose that the constitutional gain of chromosomal 14 material may have predisposed to the development of this tumor. © 2004 Wiley‐Liss, Inc.