We reviewed MR images obtained before and after the injection of Gd-DTPA in six patients with pituitary adenoma extending into the sphenoid sinus, and correlated the imaging and histological findings. The portion of each adenoma involving the sphenoid sinus and sellar floor was less intense on both T1- and T2-weighted spin-echo images and Gd-enhanced images when compared with the sellar/suprasellar solid portion of the adenoma, which showed marked contrast enhancement. Histological examination of adenoma specimens taken from the sphenoid sinus demonstrated abundant collagen fibers between the adenoma cells, whereas tumor tissue from the suprasellar region showed no fibrotic change. We concluded that pituitary adenomas seem to have a tendency to develop fibrosis on extension into the sphenoid sinus.