The purpose of the present investigation was to study the sella turcica/pituitary gland region in trisomy 21 fetuses and to relate the findings in the region to the ossification pattern in the axial skeleton formed by the cranial base and spine. Material from 22 human fetuses with trisomy 21, CRL 80 mm to CRL 190 mm, corresponding to gestational ages from 14 to 21 weeks, was examined and compared with material from gestation-matched normal controls. After radiography, tissue blocks from the cranial base, including the pituitary gland, were examined and compared with those of normal fetuses. Four different types of sella turcica/ pituitary gland morphology were observed. Thirteen fetuses (Type I) were morphologically normal. Minor abnormalities occurred in the sella turcica and pituitary gland (adenopituitary gland tissue pharyngeally) in six fetuses (Types II and III). There was agreement between the histologically recorded deviations in the sella turcica and the radiographic observations of the basisphenoid bone. In three cases (Type IV) out of 22, more pronounced structural abnormalities occurred in the sella turcica, and radiographically the basisphenoid bone appeared cleft. All sella turcica changes observed in trisomy 21 were situated anteriorly in the base of the sella. In all cases the basilar part of the occipital bone was normal. Minor changes in the sella turcica region were mainly accompanied by cervical vertebral abnormalities, while the most severe abnormalities occurred in association with malformations in the lumbar vertebrae. There was no association between sella turcica malformations and the absence or presence of the nasal bone.