To assess the prevalence of osteoporosis, bone mass measurement was performed on 1,047 women attending a menopause clinic at Chulalongkorn Hospital, Bangkok. The mean age of the population was 50.5 +/- 5.7 years. The bone density was measured at lumbar spines (LS) (L1-L4) and the non-dominant femoral neck (FN) site utilizing a Hologic QDR 2000 dual energy X-ray absorptiometer. According to the World Health Organization's (WHO) definition a value of bone mineral density (BMD) that is more than 2.5 standard deviation (SD) below the young adult mean is considered diagnostic of osteoporosis. In this study, Thai and American cut-off values of BMD for osteoporosis were used to compare the prevalence of osteoporosis. Using Thai's cutoff value, the results showed a lower prevalence of osteoporosis of both LS and FN (15.7% and 9.5%, respectively). Considering the subgroups of the studied population, the prevalence of osteoporosis of LS and FN utilizing Thai's cutoff value was significantly higher in postmenopausal than in premenopausal women. (Premenopause vs postmenopause, LS: 4.7% vs 21.4%; FN: 4.7% vs 11.9%, respectively, P < 0.05) WHO's definition of osteoporosis (the cutoff value of 2.5 SD below the young adult mean) is based on the rationale that this cutoff value identifies approximately 30 per cent of postmenopausal white women as having osteoporosis which is approximately equivalent to the lifetime risk of fracture at the spine, hip and forearm of white women at age 50 years. The prevalence of osteoporosis obtained in this study might not represent the true magnitude of the problem in Thailand. Until we have our own lifetime fracture risk which will enable us to have an appropriate cutoff value to diagnose osteoporosis, this prevalence might be used as an approximate figure or initial information for further research in this field.