The abundance of endothelial cells in bone marrow and the proximity of these cells to osteoclasts and osteoblasts suggest a role for endothelin-1 (ET-1) on bone metabolism. In vitro, the direct contact with bone endothelial cells induces osteoclastic progenitors to differentiate into mature elements. Recently it has been reported that ET-1 inhibits osteoclastic bone resorption and cell mobility through a specific receptor on osteoclasts; other authors demonstrated that ET-1 exerts a mitogenic activity on osteoblast-like cells (MC3T3) by stimulating tyrosin phosphorylation. We measured ET-1 circulating levels in patients with active Paget's bone disease, a condition with accelerated bone turnover. For the study we recruited 11 patients with Paget's bone disease (5F, 6M; mean age 68.2 +/- 3.6) in the acute stage of the disease; 10 healthy subjects (7F, 3M; mean age 66.5 +/- 3.9) were also enrolled as controls. Plasma ET-1 levels were measured with RIA kits provided by Nichols Institute. Patients showed significantly (P < 0.01) higher ET-1 circulating levels than controls (6.35 +/- 1.9 versus 3.4 +/- 1.2 pg/ml) with a positive correlation (r = 0.63; P = 0.038) with serum alkaline phosphatase (ALP), but not with urinary hydroxyproline. The higher levels of ET-1 in our patients suggest a physiopathological role for this peptide in the disease and, could perhaps represent a new useful marker of Paget's bone disease activity.