Accurate recognition of Gleason pattern 5 (GP5) prostate adenocarcinoma on needle biopsy is critical as it is associated with disease progression and adverse clinical outcome. Despite important implications of this diagnosis, interobserver variation in the diagnosis of GP5 has not been adequately studied. Digital images of 66 prostate adenocarcinoma cases that potentially contained a GP5 component were distributed to 16 urologic pathologists who were asked to classify whether GP5 was present. Each image was initially classified into 1 of 4 morphologic subpatterns by 2 coauthors (R.B.S. and M.Z.): solid nests (15), comedocarcinoma (8), single cells and/or cords (35), and variant morphology (8). Additional features captured included: size (large: >20 cells, medium: 10 to 20 cells, and small: <10 cells) and distribution of nuclei (uniform vs. nonuniform) for nests pattern; intraluminal coagulative tumor necrosis, karyorrhectic debris, and amorphous material for comedocarcinoma pattern; and quantity (≤5, 6 to 10, and >10) and distribution (clustered vs. intermixed with adjacent well-formed glands) for single cells/cords pattern. Interobserver reproducibility of a diagnosis of GP5 was assessed and the morphologic subpatterns and features were correlated with the consensus diagnosis (defined as 75% agreement). Interobserver reproducibility for overall diagnostic agreement was fair (κ=0.376). Among subpatterns, comedocarcinoma had highest reproducibility (κ=0.499), followed by variant morphology (κ=0.443), single cells/cords (κ=0.369), and nests (κ=0.347). All cases with the following features achieved consensus for GP5: large nests regardless of nuclear distribution; coagulative necrosis with or without karyorrhectic debris; single cells/cords >10 or 6 to 10 in a cluster; and signet ring-like cells in single cells or within nests pattern. A majority of cases with the following features achieved consensus against GP5: medium-size nests; exclusive intraluminal amorphous material; single cells/cords ≤5; and Paneth cell change. Remaining morphologic features did not reach consensus for or against GP5. A majority (86%) of participants would diagnose a small focus of GP5 only when it is present in >1 level. The diagnostic reproducibility of GP5 within certain morphologies was only fair among urologic pathologists. However, the diagnosis of GP5 was more reproducible when certain restrictive morphologic and quantitative criteria were applied. These findings suggest that additional studies are needed to find highly reproducible features of GP5 associated with documented aggressive clinical outcome.