The presence of a Gleason pattern 5 prostatic adenocarcinoma is associated with a worse outcome. This study assesses the accuracy of grading a tumor as having Gleason pattern 5 and the potential factors contributing to its undergrading. From the consultation service of one of the authors, we identified 59 consecutive needle biopsy cases comprising 138 parts that, upon review, were graded as having Gleason pattern 5. All cases were reported as the final diagnosis by the outside pathologist. They were sent for a second opinion at the behest of clinicians or patients and not because the pathologist was seeking a second opinion. Considering the highest Gleason score in a given multicore specimen as the overall Gleason score, Gleason pattern 5 was missed in 34 of 59 (57.6%) cases by the outside pathologist. Compared with the outside pathologist's diagnosis, the Gleason score rendered at the second opinion was increased in 101 of 138 (73.2%) parts, was decreased in 5 of 138 (3.6%) parts, and remained unchanged in 32 of 138 (23.2%) parts. Gleason pattern 5 was not identified by the initiating pathologist in 67 of 138 (48.6%) of the evaluated parts. The architectural patterns of pattern 5 were as follows: single cells (n=104, 75.3%); solid sheets (n=69, 50%); cords (n=62, 44.9%); and comedonecrosis (n=3, 2.2%). Pattern 5 was missed more frequently when it was not the primary pattern. The most common Gleason pattern 5 architectural type was single cells and the least common was comedonecrosis. None of the architectural patterns appeared to be more correctly identified than the others; however, the most accurate grading was when the primary pattern was 5 and was composed mostly of solid sheets. Owing to the important prognostic and therapeutic implications of Gleason pattern 5, pathologists must be attuned to its varied patterns and to the fact that it may often represent a secondary or tertiary component of the carcinoma.