Active surveillance (AS) has been claimed to avoid overtreatment of prostate cancer (PCa). It remains unclear which patients may benefit from AS. One way to clarify this is to improve the definition of insignificant PCa. PSA and Gleason score—the basic instruments used to select patients for AS—suffer from systematic errors. The nomograms used to define insignificant PCa are based on patients whose disease was classified before changes were introduced in the 2005 Consensus Conference on Gleason Grading; thus, the experience obtained cannot be directly applied to today's patients. Additionally, despite the standardization of prostate-specific antigen assays promoted by the World Health Organization, differences persist and could lead to misclassification of patients. These factors lead to an incorrect classification of patients into risk groups. Although new variables would increase risk group classification, the necessary first step is to optimize the use of both prostate-specific antigen serum levels and Gleason score.