Introduction: Multiple myeloma (MM) is characterised by an uncoupled bone formation/resorption process resulting in osteolysis. Osteocalcin (OC) and bone-specific alkaline phosphatase (BAP) are markers of osteoblastic activity, whereas pyridinoline products and the cross-linked aminoterminal of type 1 collagen (NTx) reflect bone destruction. In this study, these markers were studied in relation to bone disease severity and other clinical parameters of MM activity. Methods: Serum calcium, creatinine, CRP, beta 2 microglobulin (b(2)M), M-component, OC, BAP and free urine pyridoline (Pyd) and deoxypyridinoline (Dpd), free urine Dpd and NTx were determined in 38 newly diagnosed MM patients. X-ray examination defined the degree of bone involvement. Patients were classified according to the Duric-Salmon staging system. Results: NTx, free urine Pyd+Dpd, and free urine Dpd increased with increasing degree of bone involvement. NTx was significantly higher in stages II and III compared to stage I (mean values: 100.7, 163.5 and 208.3 nmol BCE/mM creat, respectively, p < 0.002). Free urine Pyd+Dpd correlated positively with b(2)M and CRP. OC was increased in stages I and II compared to III (p < 0.005) and was inversely correlated with NTx, free urine Pyd + Dpd, and free urine Dpd alone. Conclusions: The measurement of bone turnover markers in MM provides significant information regarding disease progression and should be included in the evaluation of MM patients. (C) 2002 Elsevier Science B.V. All rights reserved.