Prostate cancer is classified into different grades of aggressiveness using the Gleason system. This system assigns a Gleason grade ranging from 1 through 5 based on how closely the cancer cells resemble normal prostate cells.

Because prostate cancers often have areas with different grades, a grade is assigned to the two areas that make up most of the cancer. These two grades are added to give a Gleason score between 2 and 10. The higher the Gleason score, also referred to as the Gleason sum, the faster the cancer is likely to grow and the more likely it is to spread beyond the prostate.