Dual-energy x-ray absorptiometry (DXA) is widely used for identifying patients with osteoporosis, making decisions about the commencement of preventive therapy, and following up response to treatment. It is important that radiologists and nuclear medicine physicians issuing clinical reports present clear interpretations that aid the primary care physician in making decisions affecting treatment. This review discusses the principles behind the interpretation of bone mineral density (BMD) studies. After a World Health Organization report published in 1994, osteoporosis is often diagnosed on the basis of the patient's T-score value (difference of BMD from young adult mean normalized to the population SD). T-scores are a measure of current fracture risk. There are problems relating to the use of T-scores in the elderly, and we argue that decisions about treatment are generally best made on the basis of the Z-score value (difference of BMD from age-matched mean normalized to the population SD) because this measures the patient's fracture risk relative to his or her peers. Recent studies confirm that the posteroanterior (PA) projection lumbar spine scan is still the optimum measurement site for monitoring response to treatment. A BMD change of 4.5% is required to register a statistically significant change.