A number of anatomic structures and pathophysiologic processes can mimic small-acinar carcinoma of the prostate gland. Seminal vesicles, ejaculatory ducts, and Cowper's glands can, at times, enter into the differential diagnosis of adenocarcinoma. Likewise atrophy, postatrophic hyperplasia, and some variants of central nodular hyperplasia can present a troubling small-acinar pattern. Another common proliferation referred to as atypical adenomatous hyperplasia lies on a morphologic continuum with low-grade acinar carcinoma and is thought by many investigators to be premalignant. Its distinction from minimal-deviation adenocarcinoma is made on the basis of both architectural and somewhat arbitrary nuclear criteria. When encountered alone, atypical adenomatous hyperplasia alerts the pathologist to examine all tissue, as it is a frequent accompaniment of well-differentiated adenocarcinoma. Once a diagnosis of low-grade acinar carcinoma has been made, accurate pathologic staging, especially in conjunction with clinically unsuspected disease (stage A), is extremely important in treatment planning and prognostication.