A 53-year-old woman presented with severe left retro-orbital pain and diplopia for 1 month [Figure 1]. Neurological examination performed on admission in a local hospital revealed left abducens nerve palsy. A magnetic resonance image (MRI) revealed an enlargement of the bilateral CS, in which the left side was dominant [Figure 2a]. Biochemical analysis of the blood and cerebrospinal fluid revealed no systemic or neuronal inflammation. A presumptive diagnosis of THS was made. Neurological symptoms were relieved by two sessions of steroid pulse therapy with methylprednisolone (mPSL, 1000 mg/day for 3 days). She began receiving regular corticosteroid therapy with 30 mg/day of oral mPSL. However, right retro-orbital pain and right abducens nerve palsy developed over a period of 3 weeks in spite of the steroid administration. Therefore, the oral mPSL was increased to 60 mg/day. Her symptoms worsened, and she was transferred to the neurological department in our hospital for further examination and treatment. There were leukocytosis, a slightly high level of C-reactive protein, and an increased erythrocyte sedimentation rate. The blood coagulation function, autoimmune test, infection examinations, and tumor markers were normal. The patient had bilateral retro-orbital pain and right oculomotor and bilateral abducens nerve palsy. An MRI showed noticeable enlargement of the right CS [Figure 2b]. The serum level of the pituitary hormones was within normal limits. Additional mPSL pulse therapies were introduced twice. The left abducens nerve palsy disappeared, and the right oculomotor and abducens nerve palsy were ameliorated. A gadolinium-enhanced T1-weighted MRI revealed multiple low-intensity lesions in the pituitary gland and the bilateral CS. Mucosal thickening and fluid collection in the sphenoid sinus were also seen [Figure 2c]. The amount of steroid was gradually decreased, and her orbital pain became worse. An MRI taken 1 month later showed that the inner membrane of the bilateral CS had become unclear. Hypertrophic dura mater around the sellae was also observed, and the sphenoid sinusitis had worsened [Figure 2d]. A tiny aneurysm was discovered to have arisen from the C4 portion of the right ICA as revealed by MR angiography (MRA) [Figure 3b]. This aneurysm was not apparent on the initial MRA [Figure 3a]. The follow-up hormonal examinations indicated total anterior hypopituitarism. Clarithromycin and carbocisteine (400 mg/day and 1500 mg/day, respectively) were administered for the diagnosis of bacterial sinusitis. An endoscopic endonasal transsphenoidal surgery was performed to exclude a CS tumor, such as a lymphoma, or a specific granuloma, as well as the possibility of an infectious disease; during the procedure, the sphenoid sinus and intrasellar contents were drained. A cream-like purulent content was discharged at the opening of the anterior wall of the sphenoid sinus. Mucosal thickening of the sphenoid sinus was also observed. The sellar floor was partially destroyed, and the dura mater was thick and hard. A biopsy of the hypophysis was performed. The pituitary gland was fragile, and the purulent discharge was similar to that from the sphenoid sinus. The dead space was irrigated with copious amounts of saline. Specimens were extracted from the left CS because the aneurysm was in the right CS. Histological examination of the CS showed granulomatous inflammation. This lesion consisted of eosinophilic foamy histiocyte-like cells that were positive for CD68 and negative for S100, CD1a, and inflammatory cells. The sphenoid sinus mucosa suffered from infiltration of lymphocyte and plasma cells in the edematous stroma. Inflammation was also seen in the hemorrhagic and necrotic pituitary gland. Neoplasms such as lymphoma, tuberculosis, and sarcoidosis were excluded. No bacterial growth was observed in the cultures of the drained contents. The severe painful ophthalmoplegia subsided immediately after the operation. She was diagnosed as having a steroid-resistant THS, and methotrexate (MTX, 4-8 mg/week) was administered. Clarithromycin and carbocisteine were continued for 3 months after the operation, at which time, decreased swelling of the bilateral CSs and no effusion in the sphenoid sinus were noted on the follow-up MRI [Figure 2e]. However, the aneurysm had slightly enlarged. Right ophthalmoplegia did not recur, and the aneurysm was carefully observed without any treatment. The mPSL was gradually decreased. MTX was maintained for 6 months, and the ophthalmoplegia disappeared. An MRI revealed the decreased size of the CS and pituitary lesions [Figure 2f]. However, the intracavernous aneurysm remained without change as revealed by MRA [Figure 3c]. Hormonal examination 3 months after the operation revealed still total anterior pituitary dysfunction. Hormone replacement therapy was started with hydrocortisone and levothyroxine sodium hydrate. Somatropin was also administered approximately 1 year after the operation.