Serum concentrations of interleukin-6 (IL-6), IL-6-soluble receptor (sIL-6R), IL-6 gp130-soluble receptor (sgp130), ligand of receptor activator of nuclear factor (NF)-kappaB (RANKL), and osteoprotegerin (OPG) were determined in 42 patients with polyostotic Paget's disease of bone (PDB) and acquired resistance to clodronate (M/F ratio 23:19; mean age 58.5 +/- 9.4 years) in acute phase of disease and after oral risedronate treatment (30 mg/day for 8 weeks). At baseline, pagetic patients showed higher levels of OPG, sIL-6R, and IL-6 with lower levels of sgp130 compared to 24 age- and sex-matched controls (respectively, 4.69 +/- 1.27 vs. 2.87 +/- 0.54 pmol/L; 40.89 +/- 8.61 vs. 30.98 +/- 4.24 ng/ml; 3.59 +/- 0.97 vs. 1.8 +/- 0.9 pg/ml; 327.34 +/- 43.41 vs. 411.7 +/- 79.5 ng/ml). Response to treatment is related to a significant increase of OPG levels in all patients (from 4.69 +/- 1.27 to 5.48 +/- 1.31 pmol/L). The disease remission, that is, total alkaline phosphatase (tALP) levels within the normal range after therapy, was associated with a simultaneous increase in OPG and sgp130 levels. In patients with tALP higher than the normal range after therapy, the OPG increase was associated with a parallel increase in RANKL levels. Our data suggest that serum levels of components of RANKL/OPG and IL-6 systems, before and after treatment, may be used to better define a therapeutical strategy in pagetic patients.