High-grade prostatic intraepithelial neoplasia (PIN) is characterized by cellular proliferations within pre-existing ducts and glands with cytologic changes mimicking adenocarcinoma, including prominent nucleoli, but lacking stromal invasion. To determine the architectural spectrum of high-grade PIN, 60 serially sectioned radical prostatectomy specimens with PIN and cancer were reviewed. Four common patterns of high-grade PIN were identified, usually with multiple patterns in each case: tufting (in 87% of cases), micropapillary (in 85% of cases), cribriform (in 32% of cases), and flat (in 28% of cases). Tumor grade was not significantly associated with any pattern of PIN. Luminal cytoplasmic apical blebs were found in all cases regardless of the pattern of PIN. A variety of associated architectural and cytologic features were observed with high-grade PIN: epithelial arches (in 60% of cases), cellular trabecular epithelial bars (in 22% of cases), "Roman" bridges (in 30% of cases), partial gland involvement (in 82% of cases), basal cell layer disruption with glandular budding (in 23% of cases), large cystic gland involvement (in 10% of cases), involvement by nodular hyperplasia (in 5% of cases), microcalcifications (in 8% of cases), proteinaceous luminal secretions (in 62% of cases), corpora amylacea (in 55% of cases), exfoliated cells of PIN (in 42% of cases), luminal crystalloids (in 3% of cases), and mucinous metaplasia (in 2% of cases). High-grade PIN exhibits a variety of architectural patterns while retaining the distinctive cytoplasmic apical blebs and diagnostic nuclear and nucleolar features. Identification of high-grade PIN warrants a further search for invasive carcinoma, but should not influence or dictate decisions regarding definitive therapy.