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This study describes bone mineral density (BMD) and the prevalence of osteoporosis in women and men between 30–89 years in an unselected population. BMD was measured in g/cm2 at total hip and femoral neck by dual-energy-X-ray absorptiometry in 3,094 women and 2,132 men in the 2001 Tromsø Study. BMD levels were significantly explained by age and declined progressively in both sexes from middle into old age, with highest decline in women. With osteoporosis defined as a T-score of two and a half standard deviation below the young adult mean BMD, the prevalence at the total hip in subjects above 70 years was 6.9% in men and 15.3% in women, respectively, using the Lunar reference material for T-score calculations. The prevalence increased significantly to 7.3% in men and 19.5% in women, when T-scores were calculated on basis of the young adult mean BMD (age group 30–39 years) in the study population. At the femoral neck, prevalence of osteoporosis increased from 13.5 to 18.5% in men, and from 20.4 to 35.2% in women above 70 years, respectively, depending on how T-scores were calculated. The study highlights the challenges with fixed diagnostic levels when measuring normally distributed physiologic parameters. Although BMD only partly explains fracture risk, future studies should evaluate which calculations give optimal fracture prediction.