Calcification in the stroma of adult telencephalic choroid plexus was studied of 20 postmortem brains and one biopsy by light microscopy, transmission and scanning electron microscopy and compared with calcification in psammoma bodies in normal arachnoid, five spinal meningiomas and in calcospherites of six pineal glands. Fifteen fetal and newborn choroid plexuses were also examined by light microscopy. Calcium deposition was observed in the subepithelial regions of the adult choroid plexus, in the walls of blood vessels but was mostly seen in spherical psammoma bodies. Collagen whorls 20-60 microns in diameter and surrounded by arachnoid cells, were observed in the stroma of the choroid plexus; calcium, phosphorous and iron were deposited in the collagen whorls to form psammoma bodies. Matrix vesicles and spicules resembling hydroxyapatite were associated with the arachnoid cells surrounding the collagen whorls and with the collagen fibres within the whorls. The dense amorphous calcified core of each psammoma body was surrounded by an outer coating of entwined collagen fibres readily visible by scanning electron microscopy. Similar psammoma bodies were occasionally observed in normal arachnoid. Psammoma bodies in meningiomas resembled those in the choroid plexus stroma. Calcospherites in the pineal differed from psammoma bodies; they were lobulated, more irregular in shape and did not have a collagen base. The results of this study suggest that psammoma bodies in the choroid plexus, as in meningiomas, form by a process of dystrophic calcification associated with arachnoid cells and collagen fibres. The presence of iron in the choroid plexus psammoma bodies may be a result of haemorrhage into the stroma. The mechanism of calcification in pineal remains unclear.