This study addresses two issues regarding prostatic atrophy: (1) the histologic features of atrophy as seen on needle biopsy results and how they affect the diagnosis of atrophy and (2) the cellular kinetics of atrophy and what it suggests about the mechanism of atrophy. We reviewed hematoxylin and eosin sections for 103 prostate needle biopsy specimens with atrophy. Each biopsy specimen was classified as either simple atrophy (large atrophic glands without crowding: 53 cases) or postatrophic hyperplasia (PAH) (crowded focus of small atrophic acini: 50 cases). Cell proliferation in both the atrophic and benign glands was evaluated in 103 cases by immunohistochemistry using antibodies against MIB-1. The TdT-mediated dUTP-biotin nick-end labeling technique was performed on 61 cases to quantitate apoptosis in atrophic and benign glands. Thirty-two percent of cases showed chronic inflammation, 21% showed acute inflammation, 14% showed nucleoli, and 1% showed mitoses. In comparison to simple atrophy, PAH contained more frequent prominent nucleoli (p < 0.0001) and acute inflammation (p < 0.0001), yet not chronic inflammation. In a multivariate analysis, acute inflammation and PAH pattern influenced the presence of prominent nucleoli. Staining for MIB-1 was greater in atrophic (27.5 cells/1000 cells) than in benign glands (3.5 cells/1000 cells), greater in PAH than in simple atrophy (p = 0.0015), and greater with acute (p = 0.05) but not chronic inflammation. In a multivariate analysis, only the pattern of atrophy and not acute inflammation was found to influence MIB-1. The rate of apoptosis was negligible in both the benign and atrophic glands, did not vary with pattern of atrophy, and did not correlate with MIB-1. Despite the atrophic appearance, atrophic glands in PAH show more proliferative activity than benign, nonatrophic glands and show no evidence of active involution, justifying the term "postatrophic hyperplasia" for this pattern of atrophy. Prominent nucleoli are seen more frequently in postatrophic hyperplasia, even in the absence of acute inflammation. To avoid a potential erroneous diagnosis of cancer, a constellation of features suggestive of malignancy should be considered, rather than relying on prominent nucleoli as the sole criteria for the diagnosis of prostate cancer.