We measured transforming growth factor (TGF)-beta-dependent biomarkers in plasma and in peripheral blood mononuclear cells (PBMCs) to identify suitable pharmacodynamic markers for future clinical trials with TGF-beta inhibitors. Forty-nine patients with bone metastasis were enrolled in the study, including patients with breast (n=23) and prostate cancer (n=15). Plasma TGF-beta1 levels were elevated in more than half of the cancer patients (geometric mean 2.63 ng ml(-1)) and positively correlated with increased platelet factor 4 (PF4) levels, parathyroid-related protein (PTHrP), von Willebrand Factor (vWF) and interleukin (IL)-10. PBMC were stimulated ex vivo to determine the individual biological variability of an ex vivo assay measuring pSMAD expression. This assay performed sufficiently well to allow its future use in a clinical trial of a TGF-beta inhibitor.