World Health Organization (WHO) criteria using T-scores for classifying patients as normal, osteopenic, or osteoporotic are based on bone mineral density (BMD, g/cm2) of the lumbar spine and hip and bone mineral content (BMC) (BMC, g) at the distal and midradius. There is no consensus on whether other forearm regions of interest (ROIs) can be used with the WHO criteria. Because the ultradistal radius region of interest (UDR) has a greater ratio of trabecular to cortical bone than midshaft portions of the radius, it is possible that more patients would be classified as osteoporotic if the UDR is measured. The objective of this study was to determine the prevalence of osteoporosis when using T-scores from the UDR in addition to PA lumbar spine, proximal femur (hip), and the radius 33% ROI. Retrospective data were obtained from three centers with differing patient demographics, thus reducing bias as a result of patient characteristics. Data were used only from patients who had a spine, hip, and forearm scan on the same day. Central dual-energy X-ray absorptiometry (DXA) systems included a GE Lunar DPX-L, DPX IQ, and Prodigy and a Hologic Delphi. Hologic data were for the ultradistal radius + ulna ROI (UDRU). Diagnostic classification (using the WHO T-score criteria) was made excluding and including the UDR and UDRU T-scores, in addition to lumbar spine (L2-L4 or L1-L4), hip (femoral neck, greater trochanter, or total), and the radius 33% ROI. The lowest T-score from any ROI determined the classification. For all GE Lunar patients (n = 409 women; age range: 20-96 yr), the distribution of normal, osteopenic, osteoporotic not using the UDR was 94 (23%), 170 (42%), and 145 (36%), respectively. The distribution when using the UDR was 67 (16%), 137 (33%), and 205 (50%), respectively. The difference in the ratio of normal + osteopenic versus osteoporotic when excluding and including the UDR T-scores was significant (p < 0.0001; two-tailed Fisher's exact test). For all Hologic patients (n = 153 women; age range: 44-93 yr), the distributions were 32 (21%), 66 (43%), and 55 (36%) not using and 31 (20%), 64 (42%), and 58 (38%), respectively, using the UDRU (not statistically significantly different). The group mean T-scores were lowest for the UDR compared to the spine and hip with GE Lunar but not Hologic patients.