Plasma levels of platelet-derived growth factor (PDGF) were measured in 58 female patients with breast cancer and in 9 normal female control subjects by means of a specific radioimmunoassay. Levels in normal control subjects were all below the lower limits of detection by the assay (1.56 fmol/100 µl plasma). Two of 17 (12%) patients with stage 2 breast cancer had detectable plasma levels. Among patients with Stage 4 breast cancer 13/41 (32%) had significantly elevated levels (> 2 times the lower limit of sensitivity of the assay). Patients with elevated PDGF levels had a significantly greater degree of metastatic involvement and significantly shorter survival. Apart from being a marker of aggressive high bulk breast cancer, PDGF may be involved in the acceleration of growth of some metastatic breast tumors.