A bone island is a hamartomatous malformation in the bone, unusually having a huge expansion. We report a rare case of giant bone island of femur concurrent with ipsilateral femoral head necrosis. Although roentgenographic features indicated the possible diagnosis of bone island, there were unusual findings, such as a huge expansion of the lesion, increased uptake of radionuclide on bone scan, and presence of symptoms. Cementless hemiarthroplasty was performed for femoral head necrosis and the diagnosis was confirmed by histology. Prosthesis insertion was considerably complicated by the presence of giant bone island located at the cut surface of the femoral neck. Despite concerns over the stability of the cementless prosthesis surrounded by bone island, roentgenograms at 4 years after surgery showed no evidence of loosening and the patient's symptoms completely disappeared.