Carcinoma in situ, dysplasia, prostatic intra-epithelial neoplasia, duct-acinar dysplasia and large-acinar hyperplasia are various terms describing more or less identical forms of prostatic epithelial atypia. The precancerous nature of these lesions can be demonstrated by: morphological and functional similarities with carcinoma, a younger age than that of carcinoma, a higher incidence in cancerous prostates, an identical zonal distribution and a significant progression of high-grade lesions towards carcinoma. These hypoechoic lesions can be detected or monitored by transrectal ultrasonography. They also secrete PSA at levels intermediate between those of benign prostate and adenocarcinoma. Because of the occasional risk of malignant transformation and a frequent association with carcinoma, these lesions should be regularly monitored by digital rectal examination, PSA assays and possibly by ultrasound-guided biopsies.