Early diagnosis of osteoporosis is difficult. Bone density measurements, however, are helpful in screening patients at risk. Technology now provides means of measuring densities of both peripheral and central bones. The most practical method is dual X-ray densitometry, which is of good sensitivity and accuracy. Single photon absorptiometry on peripheral bones can be used to screen populations at risk. Computed tomography is also useful but the radiation dose is relatively high. In dual X-ray densitometry, calibration methods and comparisons with reference values are fairly reliable. The best sites for measurements are the lumbar spine and proximal femur, in which osteoporotic fractures are common. In general, however, one measurement is not enough. Biological variation is great. Repeated measurements will allow estimation of the rate of bone loss. Precise location of a previous site is sometimes difficult on reexamination and extraskeletal calcifications can be sources of error. Bone densitometry in connection with the prevention and treatment of osteoporosis is fairly easily performed with a single patient. Knowledge about the correlation between bone density and fracture risk is not yet adequate. The results of measurement of bone mineral density relate only to mineral content, not bone quality, i.e., differential diagnosis of osteoporosis and osteomalacia is not possible.