We introduce a label-free method for detecting high-grade prostatic intraepithelial neoplasia (HGPIN) in unstained biopsies. We image this condition based on the identification of basal cells in biopsies that otherwise would resemble prostate cancer by unassisted histologic examination. Gradient field microscopy (GFM) is used as a label-free imaging method which increases the contrast of a transparent sample by taking the first-order phase derivative that is very sensitive to rapid refractive index changes. GFM is able to image the basal cell layers in HGPIN biopsies because of their rapid refractive index changes at the periphery of small glandular structures.