A novel method  to predict the probability of success of prostate cancer treatments with HIFU immediately after treatment was developed. It is based on an acoustic energy density measure computed by dividing the energy delivered during a HIFU treatment by the prostate gland volume. This method has been implemented in the Sonablate®500 (SB-500) HIFU prostate cancer device (Focus Surgery, Inc., Indianapolis, USA). Its utility was verified by correlating the energy density index to the 180 day post-treatment biopsy results for each subject enrolled in a Phase I clinical trial (N=19) for the primary treatment of organ confined prostate cancer with the SB-500. For each HIFU treatment and at each treatment site the total acoustic power (TAP) and the HIFU "ON" time were recorded. This information, along with the dimensions of the prostate prior to HIFU treatment and an estimated tissue attenuation coefficient were used to compute the energy density measure. Two values for the attenuation coefficient at 4 MHz were used to estimate the energy absorbed by the prostate: infinite attenuation (α=∞) and finite attenuation (α=0.32 Np/cm). The energy density results for subjects with a negative biopsy were found to be significantly higher than the energy density results for subjects with a positive biopsy when using either infinite or finite attenuation estimates (p<0.02 for both). Receiver operator characteristics (ROC) were computed using the post-treatment biopsy as a gold standard. The area under the ROC curve was found to be 0.77 and 0.80 for infinite and finite attenuation value. This developed energy density measure will not replace current HIFU prostate cancer treatment success indicators (i.e. biopsy), but its ability to be computed immediately post-HIFU treatment and its high correlation with clinical success indicators makes it a useful measure of treatment success.