Pituitary collision tumors are rare. They may create difficult diagnostic problems and their histogenesis is not clear. We report here an unusual case of a somatotroph adenoma colliding with a gonadotroph adenoma.The 64-year-old man had clinical acromegaly. His blood growth hormone level was elevated and magnetic resonance imaging demonstrated a pituitary tumor. The surgically removed sellar mass was investigated by histology, immunocytochemistry and electron microscopy. Morphologic study revealed a collision tumor; one was a somatotroph adenoma, the other a gonadotroph adenoma. Authors call attention to the difficulties in clinical, imaging and pathological diagnosis. Detailed morphologic studies are needed to establish the presence of two distinct tumors composed of two different cell types.