Lymphedema of the arm is one of the most disabling and serious complications of breast cancer. Apart from tumor infiltration or fibrosis of lymphatic pathways, little is known about factors favoring the development of lymphedema. In the present study, we investigated the impact of rheologic parameters, e.g. red cell aggregation (EA) and plasma viscosity (PV), and of capillary morphology and capillary flow in patients with breast cancer with (n = 18) and without (n = 18) lymphedema. Patients with lymphedema showed a significant increase of red cell aggregation (p < 0.001) that indicates a systemic component of lymphedema and might offer a possibility of prevention and therapy of this condition. A hitherto unclassified protein factor favoring red cell aggregation and lymphedema might be postulated.