A system with combined therapy/imaging capability for noninvasive lesion formation in tissue using high-intensity focused ultrasound will be presented. The system combines a highly focused therapeutic device with an imaging transducer for monitoring the treated volume in realtime using signal processing algorithms of ultrasound radio frequency (RF) echo data. Preliminary results indicate that it is feasible to distinguish between reversible and irreversible change in the state of the tissue due to high-intensity focused ultrasound. An autoregressive model has been assumed for processing echo data from the treated region. The reflection coefficients for a low-order AR model are calculated from A-line data taken during the treatment (every 1 to 2 seconds). The contours of the reflection coefficients can be used to distinguish between reversible and irreversible change in the state of the tissue. Noninvasive quantitative measures of thresholds for irreversible tissue damage based on the AR reflection coefficients are currently being investigated. Experimental results from our investigation will be presented. In addition, a review of the current status of research on tissue ablation using high-intensity focused ultrasound will be given.