We report a patient with Morgagni syndrome. The main aim of this paper is to discuss hyperostosis frontalis interna (HFI) and coexisting clinical feature and to describe the pathomorphology in detail on the basis of MRI images of the skull. The patient, a woman, was 82 years old when she first came to our hospital. She had a 20-year history of hypertension and chronic headache, and had been excessively obese till three years before. On admission she presented with a broad spectrum of clinical symptoms and signs including insomnia, disorientation to place, loss of memory, dementia, night delirium, reduced deep tendon reflexes in the lower extremities, urinary incontinence and upward gaze palsy. Because of a fair recovery within several days, it was suspected that so-called "treatable dementia" played a considerable role in the above-mentioned clinical state. Laboratory testing data, including hormone levels, were all within normal limits. EEG examinations showed slowed, diffuse, and poorly developed alpha-waves with no paroxysms. Cranial CT in horizontal sections disclosed a deformed frontal bone with convexlens-shaped thickening bilaterally and diffuse high density on both sides. MRI images revealed more detailed structures: the outer plate, diploe and inner plate of the skull, and abnormal ossifications. Based on these findings we diagnosed her illness as Morgagni syndrome. Recent reports, though few in number, have tended to focus on the EEG findings, hormones and psychiatric states in this syndrome, and descriptions of the HFI itself seem to be rare. The true cause of this syndrome is not yet known, so this rare presentation of MRI images of HFI is thought to be important in explaining this peculiar phenomenon in the skull.