The aim of the study was to report values for osteoporosis (OP) prevalence in Buenos Aires. Bone mineral density (BMD) at different skeletal sites was measured from November 2012 to July 2014. Participants were recruited through a newspaper advertisement inviting women at least 50 yr of age to receive free BMD measurement. After signing an informed consent form, 5448 women living in Buenos Aires and surrounding districts were studied. Lumbar spine (L1-L4), femur neck, and total hip BMDs were measured (Lunar Prodigy, software version 12.3 GE, Madison, WI, USA). OP was defined as a T-score ≤-2.5 at the lumbar spine or the femoral neck. Results showed that 1021 out of 5448 studied subjects (18.7%) had OP at the lumbar spine or the femoral neck. Comparison of age of the population sample with reference data for the general population showed a moderate (+0.6%) increase in prevalence. Prevalence of OP was low, up to the age of 70 yr when based on femoral neck BMD only. Conversely, the prevalence of OP at the lumbar spine, which was reportedly high in women up to the age of 70 yr, tended to level off over that age. The results of the total femur only added a slight (+0.7%) nonsignificant increase to the OP prevalence. A total 346,500 out of 1,853,000 women aged 50+ yr in Buenos Aires had OP at the lumbar spine or femoral neck, whereas only 163,500 had OP at the upper femur, reducing the number by 53%. The present study assessed OP prevalence in the most densely populated urban area in Argentina. The results are similar to those reported for Caucasian populations in the United States and Canada. As measurement of only the BMD of femoral neck overlooks the diagnosis in half of the women, future studies should include measurement of the lumbar spine in combination with the femoral neck for a more accurate estimation of OP prevalence.