Osteoporosis is a disease in which low bone mass and microarchitectural deterioration of bone tissue lead to increased bone fragility and a consequent increase in fracture risk. The risk of developing osteoporosis can be assessed by determining the maximum density and strength achieved at maturity (peak bone mass) and the rate and duration of age-associated bone loss. The major cause of osteoporosis is estrogen withdrawal in women, most commonly associated with the menopause, but also with other causes of ovarian failure. Androgen insufficiency in men, although much less common, can also lead to osteoporosis. Measurements of bone mineral density (BMD) have been used to predict fractures, and current evidence suggests that fractures at any site can be predicted by taking measurements of BMD at any other site in the skeleton, using noninvasive techniques such as single or dual energy absorptiometry, quantitative computed tomography and ultrasound, a promising but experimental approach. Rapid bone loss at the start of the menopause is also an important contributing factor to the development of osteoporosis. Levels of biochemical markers of bone turnover in plasma and urine have been found to correlate with rapid and prolonged bone loss. Powerful new assays for estimating bone turnover have emerged and more are being developed. Various combinations of these biochemical tests may be used in conjunction with bone densitometry to predict future risk of osteoporosis and osteoporosis-related fractures. Furthermore, biochemical tests can also be useful in assessing response to therapy. Although many factors, including sex, race, heredity and lifestyle (e.g., calcium intake, minerals, nutrition and exercise), influence the risk of osteoporosis, i.e., they affect peak bone mass and subsequent bone loss, and are of little use in predicting future occurrence.