In a study of plasma and tissue platelet derived growth factor (PDGF) concentration in patients with breast cancer, elevated levels of plasma PDGF were found in a significant proportion, 11/37 (30%), of patients. Sixteen patients (43%) had tumors which expressed PDGF-AA and 6 patients had tumors which in addition expressed the BB isoform of PDGF. All patients with elevated plasma levels of platelet derived growth factor had tumors which expressed the growth factor on immunohistochemical staining of tumor cells. Furthermore there was a significant correlation between plasma levels of platelet derived growth factor and the intensity of tissue staining. Patients with stage four breast cancer with tumors which were positive for platelet derived growth factor had a significantly lower response rate to chemotherapy as well as significantly shorter duration of survival. In addition, patients with stage four breast cancer who had elevated plasma PDGF levels had a significantly shorter survival. These results indicate that elevated plasma levels of platelet derived growth factor in patients with breast cancer are derived from the tumor cells and suggest that platelet derived growth factor may play a significant role in control tumor cell growth.