Routine bone mineral densitometry (BMD) screening has been recommended for women aged >or=65 yr (Osteoporosis Canada [OC], International Society for Clinical Densitometry [ISCD], Canadian and United States Task Forces on Preventative Healthcare, and National Osteoporosis Foundation) and for men >or=65 yr (OC) or >or=70 yr (ISCD). We estimated the number of older Canadians needed to screen (NNS) by BMD to detect an undiagnosed case of osteoporosis, using prospective, multicenter, population-based data from the Canadian Multicentre Osteoporosis Study (CaMos). We included participants aged >or=65 yr with baseline dual-energy X-ray absorptiometry (DXA) BMDs at the femoral neck and lumbar spine (L1-L4). Osteoporosis was defined by a T-score <or=2.5 at either site. Patients were questioned about a prior diagnosis of osteoporosis. We studied 2699 women and 1032 men aged >or=65 yr. The percentage prevalence and 95% confidence intervals were determined. In individuals aged >or=65 yr, the prevalence of osteoporosis was 25.6% in women (95% confidence interval, 24.0%, 27.3%) and 8.9% in men (7.3%, 10.8%). In 652 men aged >or=70 yr, the prevalence of osteoporosis was 11.3% (9.1%, 14.0%). Of the participants with BMD-defined osteoporosis, 76.6% of woman aged >or=65 yr (73.2%, 79.6%; 516 of 674 women), 93.4% of men aged >or=65 yr (86.4%, 96.9%; 85 of 91), and 93.2% of men >or=70 yr (84.9%, 97.0%; 68 of 73) were not aware of it. Thus, the minimum NNS by BMD testing to detect one previously undiagnosed case of osteoporosis in Canada is: 6 women aged >or=65 yr, 13 men aged >or=65 yr, and 10 men aged >or=70 yr.