A one-dimensional and a three-dimensional computer model have been built in order to study the importance of blood flow and ultrasonic absorption in tissues during local hyperthermia. The decreased blood flow in the interior of certain tumours and possibly the increased ultrasonic absorption of the malignant tissue in some cases may cause selectively higher temperatures inside the tumours though the heat input is the same as in the surrounding tissues. Also, the vasodilation of blood vessels in normal tissues as a response to heat causes a therapeutically useful temperature difference. These blood flow differences can lead to enhanced effects during sonication to produce hyperthermia in the tumour. The inhomogeneity of blood flow in the tumour causes a non-uniform temperature distribution leaving the well-perfused cells in the advancing front at a much lower temperature than the cells in the necrotic centre. Thus, the combination of local hyperthermia with radio-and chemotherapy seems to offer the most attractive means of destroying malignant tissue.