Simple bone cyst (SBC) is an intra-osseous pseudocystic lesion lined by a thin fibrovascular membrane but lacks an epithelial lining. SBC is uncommon and comprises of approximately 1% of all jaw cysts. The lesion is usually discovered incidentally during the first two decades of life. Males are affected slightly more frequently than females. The most common sites of occurrence are the molar and premolar regions of the mandible; maxillary lesions are uncommon. The lesion is usually solitary, radiolucent, well demarcated, and non-expansile. Margins may be scalloped, between vital teeth, and root resorption is rare. A surgical approach to the lesion reveals an empty cavity with serosanguinous fluid. Histologically, the cavity lining consists of a loose connective tissue layer and hemosiderin-laden macrophages. Pathogenesis and etiology of SBC are still not clearly understood. It is assumed that the cyst forms following traumatic intra-osseous hemorrhage with subsequent lysis and resorption of the cellular content. Trauma, as the main cause of SBC, is not always conclusive and the origin of the lesion may be multifactorial. Treatment of SBC is by curettage. Intralesional hemorrhage due to either intentional curettage or during exploratory procedure may induce a reparative process. A rare occurrence of SBC in a 71 year old healthy male, referred to the Oral and Maxillofacial Outpatient Clinic, Sheba Medical Center by his dentist, because of a large radiolucent lesion in the left mandibular body is reported. The patient received a complete lower denture 3 months earlier. He returned to his dentist with a complaint of local pain and an ulcer under the denture. A panoramic radiograph revealed a large unilocular radiolucent lesion with demarcated borders in the left mandibular body and bone erosion in the upper alveolar border of the lesion Clinical examination showed a deep ulcer in the denture-bearing area of the left posterior alveolar ridge. There was no cortical expansion of the mandible. Incisinal biopsy revealed an empty cavity lined by fibrous connective tissue with no epithelial lining. According to the clinico-radiologic and histologic findings, a diagnosis of a simple bone cyst was made. The incidence of SBC is higher in younger age groups (second decade). Occurrence of the lesion in the elderly is rare with only a few reports in the literature. In the present case, the fact that SBC is asymptomatic, may be responsible for the late discovery of the lesion.