The prognostic significance of Paneth cell-like neuroendocrine differentiation in adenocarcinoma of the prostate has not yet been established. We studied 36 cases of adenocarcinoma of the prostate showing Paneth cell-like neuroendocrine differentiation, including needle biopsy specimens (n = 27), radical prostatectomies (n = 8), and transurethral resection specimens (n = 1). Paneth cell-like neuroendocrine cells (NECs) were observed as either patchy isolated cells or diffusely involving glands or nests. With Gleason pattern 3, a patchy pattern of NECs was seen in 18/19 cases with only 1/19 (5.3%) case showing diffuse NECs. All the 4 Gleason pattern 4 cases had patchy NECs. Of the 21 cases with Gleason pattern 5, 18 (85.7%) had diffuse NECs with the remaining 3 exhibiting patchy NECs. Radical prostatectomy was performed in 16/36 (44.4%). Tumor was organ confined in 10/16 cases (62.5%). Extraprostatic extension (EPE) with positive surgical margins was seen in 6/16 cases (37.5%). In 4 cases, seminal vesicles were positive for cancer. Pelvic lymph nodes were free of tumor in all cases. The actuarial prostate specific antigen progression-free risk at 5 years and 7 years was 92% and 80%, respectively. Only 2 patients progressed after radical prostatectomy and they both had Gleason score 7 cancer with extraprostatic extension and seminal vesicle invasion. Of the 16 radical prostatectomy cases, 8 (50%) had a Gleason pattern 5 component either on needle biopsy or at radical prostatectomy, with nests, cords, or single cells containing Paneth cell-like neuroendocrine differentiation. Five of these 6 cases with Gleason pattern 5 and available follow-up information had no evidence of progression with mean and median follow-ups of 46 months. Radiation therapy either as monotherapy or combined with hormonal therapy was used to treat patients in 13/36 cases. Overall only 2 patients progressed, one with clinical T2 and the other T3 disease. Of the 5 cases with Gleason pattern 5 composed in part or totally by NECs treated by radiation therapy, all are without evidence of recurrence with a mean and median follow-up of 47 and 45 months, respectively. Of the remaining 5 cases with available follow-up treated with watchful waiting, hormone therapy, or cryotherapy, 4 had Gleason pattern 5 tumor with NECs. Of these 4 cases, 3 had no progression with a mean and median follow-up of 42.5 and 60.5 months, respectively. Despite the cells' bland histologic appearance, strictly applying the Gleason grading system one would have to assign a Gleason pattern 5 to these foci with no glandular differentiation. The current study demonstrates that applying the Gleason score to these foci does not accurately reflect their clinical behavior. In cases with Paneth cell-like NECs, only the conventional adenocarcinoma component should be assigned a Gleason score. In cases in which the entire tumor is composed of Paneth cell-like cells and areas of the tumor lack glandular differentiation, the tumors should not be assigned a Gleason score and a comment should be provided as to the generally favorable prognosis of this morphologic pattern of neuroendocrine differentiation.