Dup (6p) patients have a peculiar facial appearance (frontal bossing, hypotelorism, hypoplastic midface), low birthweight, cardiovascular defects, small kidneys, and psychomotor retardation. We thought that a detailed anatomical dissection would more precisely define the syndrome, which has been developed from clinical evaluations and autopsy reports. Our patient, a female adolescent, died at 17 11/12 years and is the oldest patient with this syndrome to be described. The brain and skull showed the greatest number of abnormalities. The brain was smaller than normal and abnormally shaped. Many of the skull abnormalities, including shortened basisphenoid/basiocciput, reduced size of cranial fossae, and steep orbital roofs, may be attributed to the brain's shape. There were no olfactory bulbs, and only one rudimentary olfactory tract was present. Other findings were the following: a high interventricular septal defect and right ventricular hypertrophy, absence of uterus and vagina, hypoplastic ovaries, a common mesentery, two left extensor indicis bellies, bilateral absence of palmaris brevis and of peroneus tertius. Our results are compared with anomalies found in other aneuploidy syndromes. Variations in some organ systems may be similar to those whose presence Shapiro [1983] has attributed to amplified developmental instability. Relationships between the brain shape and size and skull abnormalities are also discussed. From 16 reported cases, a detailed autopsy report of the affected brother, and our anatomical findings, we suggest that this syndrome may also be characterized by arhinencephaly, common mesentery, absent uterus in the female, and growth retardation in those surviving the neonatal period.