A suitable marker of bone metabolism is either a measurable enzymatic activity specific for osteoblasts or osteoclasts or a bone matrix component released into the circulation and excreted in the urine during bone formation or bone resorption. Testing such bone markers is the most sensitive method for monitoring acute changes in bone metabolism. Guidelines for the use of assays in patient care are not yet established, largely because of the large intrapatient variation seen with the results of most assays. Bone markers also can be used as predictors of bone turnover, bone loss, and fracture risk. During the past year, bone markers have been intensively studied in the context of these issues.