Thermally-induced lesions are generally stiffer than surrounding tissues. We propose here to use the supersonic shear imaging technique (SSI) for monitoring high-intensity focused ultrasound (HIFU) therapy. This new elasticity imaging technique is based on remotely creating shear sources using an acoustic radiation force at different locations in the medium. In these experiments, an HIFU probe is used to generate lesions in fresh tissue samples. A diagnostic transducer, controlled by our ultrafast scanner, is located in the therapeutic probe focal plane. It is used for both generating the shear waves and imaging the resulting propagation at frame rates reaching 5,000 images/s. Movies of the shear wave propagation can be computed off-line. The therapeutic and imaging sequences are interleaved and a set of wave propagation movies is performed during the heating process. From each movie, elasticity estimations have been performed using an inversion algorithm. It demonstrates the feasibility of detecting and quantifying the hardness of HIFU-induced lesions using SSI.