Tartrate resistant acid phosphatase (TRAP) has been proposed as a new biochemical marker for bone resorption. We have compared this new marker, TRAP, with the classical biochemical markers of bone remodelling, serum alkaline phosphatase (sAP), serum osteocalcin (sBGP), and with the urinary hydroxyproline/creatinine ratio (uOHProl/creatinine), a routine marker of bone resorption. Serum TRAP was significantly higher in pagetic patients (n = 43) than in control subjects (n = 12) (13.02 +/- 4.7 vs 5.48 +/- 1.31 IU/L, P less than 0.001) and a significantly positive linear correlation was found between the sTRAP and uOHProl/creatinine ratio (y = 0.0051x - 0.0069, r = 0.82, P less than 0.001), between sTRAP and sAP (y = 19.3x - 85.0, r = 0.71, P less than 0.001) and also between sTRAP and sBGP (y = 0.02x + 2.23, r = 0.52, P less than 0.01). Serum TRAP levels were higher than the upper limit of normality in all our pagetic patients except for two, whose uOHProl/creatinine levels were in the normal range. We conclude that (1) sTRAP could be a parameter as sensitive as uOHProl/creatinine in the diagnosis of Paget's disease; (2) sTRAP and uOHProl/creatinine are both good markers of bone resorption; (3) the correlation found between sTRAP and formation markers (sAP and sBGP) makes sTRAP a marker of disease activity in Paget's disease of bone; (4) the assay of sTRAP is easier, faster, and of lower cost than the urinary hydroxyproline determination. We suggest that sTRAP determination could be used as a routine marker of bone resorption in Paget's disease of bone, as is the case with uOHProl determination.