The aim of this work has been to compare the usefulness of serum alkaline phosphatase (AP) and its bone isoenzyme (bAP), biochemical markers of bone formation and serum carboxyterminal cross-linked telopeptide of type I collagen (ICTP) and urinary hydroxyproline/creatinine (Hp/Cr), biochemical markers of bone resorption, in the study of patients with disease paget of bone (PDB).Patients: Serum samples from 27 patients with active PDB (11 males, 16 females, age 69 ± 6.6 years) and 26 age and sex paired controls were drawn after overnight fasting. 24 hours urine samples were also collected.Methods: AP: Hydrolisis of 4-nitrophenilphosphate, ph = 10.5;bAP : IRMA, TANDEM R-OSTASE (Hybritech Europe, Belgium). Hp: Kivirikko method; Cr: Jaffe reaction; ICTP: RIA Telopeptide (Orion Diagnostic, Finland).Results: AP and bAP levels were significantly higher in PDB patients than in control subjects: AP (KAU): PDB 30.2 ± 19.4, C 6.6 ± 1.4, p < 0.001;bAP (ng/mL): PDB 116 ± 115, C 12.7 ± 3.4, p < 0.001.A 96.15% of patients showed levels of AP and bAP higher than the upper limit of normality. ICTP and Hp/Cr levels were also significantly higher in PDB patients than in control subjects. ICTP (μg/L): PDB 4.62 ± 2.94, C 2.07 ± 0.58, p < 0.001;Hp/Cr (both in mg/dL): PDB 0.087 ± 0.045, C 0.025 ± 0.09, p < 0.001. A 92.3% and a 66.6% of patients showed respectivelly levels of ICTP and Hp/Cr higher than the upper limit of normality.Conclusions: bAP determination does not improve significantly AP quantitation in PDB. bAP would be the elected marker only in the cases in which an hepatic affectation is present. With respect to the studied biochemical markers of bone resorption, the classical hydroxyproline seems to be more sensitive than ICTP in the study of PDB.