The aim of this study was to explore a law of energy deposition of high-intensity focused ultrasound (HIFU) in various tissues and the expression of such a law. A focused ultrasound (US) tumor therapeutic system was used to apply a focused US beam to tissues both in vivo and in vitro. The formation of individual ellipsoid-shaped regions of coagulative necrosis has been observed. Results showed that the volume of the ellipsoid-shaped coagulative necrosis region was different from that of the acoustic focal region (AFR), both in vitro and in vivo. Acoustic intensities ranging from 7 x 10(3) W/cm(2) to 27.7 x 10(3) W/cm(2) and exposure times from 1 to 20 s gave volumes of ellipsoid-shaped coagulative necrosis of 0.2 to 2000 mm(3). Although the HIFU doses applied were identical, the volumes of individual ellipsoid-shaped coagulative necrotic regions varied with the structures of tissues, their functional status and the irradiation depths. Individual ellipsoid-shaped regions of coagulative necrosis induced by HIFU can be added to produce coagulative necrosis of an entire tumor. We define the individual ellipsoid-shaped coagulative necrosis produced by the US energy deposition of a single exposure as the "biological focal region" (BFR) of HIFU. This serves as the basic unit for HIFU ablation of tumors, and is plotted as a function of AFR, acoustic intensity, exposure time, irradiation depth, the tissue structure and its functional status.