The presence of ductal/acinar epithelial changes including nuclear enlargement, prominent nucleoli, chromatin alterations, and luminal complexity is an easy way to identify the disorder. Four main patterns of high-grade PIN (HGPIN) have been described: tufting, micropapillary, cribriform, and flat. In addition, variants of HGPIN have also been described. Both HGPIN and prostatic carcinoma share an increased incidence and severity with advancing age and with high rates of occurrence in the peripheral zone of the prostate. Furthermore, HGPIN and prostate cancer share genetic and molecular markers as well, with PIN representing an intermediate stage between benign epithelium and invasive carcinoma. The clinical significance of HGPIN is that it identifies patients at risk for prostatic carcinoma. With the increased use of extended biopsy protocols, clinicians are more likely to identify HGPIN and less likely to miss concurrent carcinoma.