When dentists examine a patient with osteoporosis, they should consider the condition of periodontal disease, because both osteoporosis and periodontal disease share the same symptoms such as reduction of the bone volume. Many studies have attempted to define the relationship between these two diseases. Although most studies support a positive association between them, the mechanism of correlation still remains unclear. To establish novel treatment modalities of these diseases, further studies are necessary to elucidate the detailed mechanisms of interaction between them.