It follows from the present work that the vascularization of the normal tissue present in the treatment volume limits the temperatures achieved during heat treatment of invasive ductal breast carcinoma. The temperatures can often be increased by giving fractionated heat treatment because heat treatment may reduce the cooling capacity of the normal tissue vasculature. Significant damage to supplying vessels occurs at the heat doses necessary to cause necrosis in the tumour and surrounding normal tissue, indicating that secondary cell death is an important mechanism for cell inactivation following hyperthermic treatment of breast carcinoma.