Three dimensional acoustic and thermal models to simulate superficial hyperthermia treatments for a new multielement ultrasonic system are presented. Typical power density and steady state temperatures distributions are shown. Temperature volume histograms are used to determine the relationship between tumor size and aperture size (number of active elements) needed for adequate hyperthermia. A new numerical algorithm to simulate heat transfer in perfused tissues with thermally significant (large) vascular structures is introduced. This numerical model was developed without making the usual assumptions of constant vessel wall temperature or constant vessel wall heat flux which may not always hold in hyperthermia situations. The thermal effect of a large artery-vein pair crossing the treatment volume is considered. Results showed that the new multielement system induces more uniform temperature distributions when compared with previously existing commercial systems. However, it was found that to adequately treat tumors with this device, power must always be deposited in the tumor-peripheral normal tissue. The effect of a single vessel pair, in terms of hyperthermia treatment adequacy, was local, but very detrimental.