After ultrasound diagnosis of a fetus with severe hydrocephalus and Dandy-Walker malformation, amniocentesis revealed chromosomal mosaicism for dup(lq) and del(Xq). The neonate had dysmorphic facial features, vertebral abnormalities, and pulmonary hypoplasia, and expired shortly after birth. This report confirms the poor prognosis reported for partial trisomies involving the long arm of chromosome number 1.