The aims of the present study were to clarify whether postmenopausal women with osteoarthritis (OA) of the knee show higher bone mineral density (BMD) than healthy controls, and to investigate the relationship between BMD and the Kellgren and Lawrence radiological grade in postmenopausal women with OA of the knee. A total of 674 postmenopausal women, 46-90 years of age, were enrolled in the study: 305 patients with OA of the knee and 369 healthy controls. Forearm (distal radius) BMD, measured by dual-energy X-ray absorptionetry (DXA), using a DTX-200 (Osteometer), was significantly higher in the OA group than in the control group (P < 0.001), even when adjusted for age, height, body weight, body mass index, years since menopause, and grip strength (P < 0.01). Analysis of variance (ANOVA) with Fisher's protected least significant difference (PLSD) test showed that BMD in radiological grades 2, 3, and 4 was significantly higher than that in grade 1 (P < 0.05, P < 0.001, and P < 0.05, respectively), and BMD in grade 3 was significantly higher than that in grade 2 (P < 0.01), but BMD in grade 4 was significantly lower than that in grade 3 (P < 0.05). These findings suggest that forearm BMD appears to be significantly higher in postmenopausal women with OA of the knee than in healthy controls. Although BMD may be increased in low to moderate radiological grades of OA, severe grade OA may not always develop from moderate grade OA. Some cases of severe grade OA may be associated with low BMD.