Corpora amylacea in the prostate are a frequent finding in benign acini, but are only rarely observed in adenocarcinoma. To determine the incidence and comparative histopathology of this finding, we prospectively reviewed all consecutive needle core biopsies (excluding consultations) received at Bostwick Laboratories between December 2001 and July 2003. Among 5130 cases of adenocarcinoma (34% of 15,279 total needle biopsy cases), we identified 19 (31 biopsy specimens) with corpora amylacea within cancerous acini (0.4% incidence). Patients ranged in age from 51 to 89 years (mean, 68 years). The corpora amylacea were located within cancers with Gleason pattern 3 (28 of 31 specimens), Gleason pattern 4 (one specimen), and Gleason pattern 5 (two specimens), and ranged from less than 0.1-0.3 mm in diameter. Coexistent eosinophilic proteinaceous debris was noted in all 31 specimens, luminal mucin in 19, crystalloids in 15, and collagenous micronodules in two specimens. Our results indicate that the incidence of corpora amylacea in adenocarcinoma is low, but the presence of such inclusions cannot be used to exclude malignancy.