Partial duplication of the long arm of chromosome 1 has been observed in fetal intracranial teratomas. We sonographically diagnosed a 19-week fetus with sacrococcygeal teratoma, cerebral ventriculomegaly, and cerebellar hypoplasia. Chromosomal analysis of amniocytes showed an unbalanced translocation between chromosomes 1 and 15, resulting in trisomy 1q21-->1 qter. Duplication or over expression at more than one locus on the long arm of chromosome 1 may be required for the development of an extra-gonadal teratoma.