The authors report a case of ossified prolactinoma that was identified as a calcified nodule on the roentgenograms. Histologically, the ossified tissues were diffuse, and the osteoid was mineralized in various degrees, subsequently forming bones. Adenoma cells were sparse among the ossified tissues. This is probably the first report of ossified pituitary adenoma associated with osteoid metaplasia. The pathogenesis of osteoid metaplasia in pituitary adenomas is possibly due to proliferative connective tissue resulting from degeneration of the pituitary adenoma; this connective tissue may possess osteogenic potency.