Bilateral absence of the pyramids, best assessed in transverse sections of the medulla oblongata, was found in 0.7% of 2,850 autopsies performed at the Royal Children's Hospital, Melbourne (Australia). A strong association with some forms of congenital aqueductal stenosis was observed and evaluated in further cases ascertained through other hospitals. The very strong association observed with X-linked congenital aqueduct stenosis may be useful diagnostically but must be evaluated in other centres. At least, the presence of pyramids seems strong evidence against this diagnosis. Absence of the pyramids was also observed in several known or new genetic syndromes — Meckel's syndrome, the HARD syndrome, two sisters with congenital aqueductal stenosis and polydactyly and two families with microcephaly and arthrogryposis.