Since bone markers may reflect different aspects of bone disorders and cell function, and osteolytic and osteoblastic activities may be individually or concomitantly altered, determination of more than one marker type is generally appropriate. Also, the individual markers of a particular type do not necessarily show parallelism. For example, in osteomalacia from vitamin D deficiency, bone-specific alkaline phosphatase may be grossly elevated because of enhanced osteoblastic activity, whereas the vitamin D dependent osteocalcin may be decreased. With the exception of measurement of the bone enzymes, bone-specific alkaline phosphatase and tartrate-resistant acid phosphatase, bone marker measurements require complex and expensive immunoassays. As a general rule, the simple enzyme measurements can precede other investigation in most bone disorder> Bone-specific alkaline phosphatase measurement alone is generally adequate for the investigation of osteomalacia, Paget's disease and hyperparathyroidism but should be combined with measurement of tartrate-resistant acid phosphatase in suspected metastatic disease, and in multiple myeloma. Determination of both enzymes together may also be of value in the investigation of osteoporosis but in this disorder added benefit may be obtained by the addition of other bone markers, particularly urine deoxypyridinoline and possibly serum collagen telopeptide.