An 82-year-old diabetic female had been aware of gradually enlarging, painless scalp depressions in the bilateral parietal regions for more than 6 years. She had no history of head injury, or inflammatory or malignant disease. Her family history was unremarkable for hereditary bone disease. She had diabetes mellitus which had been well controlled with orally administered drugs. Blood examination showed normal renal and liver functions with normal serum calcium, magnesium, and phosphorus concentrations, in addition to normal parathyroid hormone and calcitonin levels. Neuroimaging including skull radiography, head computed tomography, and magnetic resonance imaging demonstrated symmetric thinning in the bilateral parietal bones attributed to loss of diploe and thinning of the outer table. The inner table was intact without associated soft tissue mass or vascular lesions. Technetium-99m methylene diphosphate systemic bone scintigraphy showed mild hypoaccumulation in the affected calvarium on the left. No other pathological findings were found by systemic examination. She underwent open biopsy for histological verification which revealed homogeneous membranous bone tissue with regressed diploe, absence of osteoblasts, absence of osteoclastic appearance, and absence of findings of underlying malignancy. There was no markedly fibrovascular connective tissue typical of Gorham-Stout disease. Calvarial thinning in the present case may have been caused by an undetermined complex mechanism.