Focal osteoporotic marrow defect (FOMD) may be the earliest detectable form of the ischemic marrow disorders. The exact cause is unknown, but three theories have been proposed in the literature. A fourth is presented in this paper. In this study, 100 biopsies were examined histologically and were diagnosed as FOMD, based upon consistent histological characteristics. Until recently, the only diagnostic criteria were radiographic evaluation and incisional biopsy. In February 2002, a through-transmission alveolar ultrasonic test (Cavitat 4000, Cavitat Medical Technologies, Inc., Aurora, CO) was approved by the US Food and Drug Administration and by Health Canada for detection of low bone density and bone desiccation, both features of FOMD and chronic ischemic bone disease. Within this article, the diagnostic criteria and pathological findings of FOMD will be presented. The three current theories concerning its etiology will be briefly presented and a fourth theory will be proposed.