To determine whether p53 immunoreactivity correlates with the Gleason tumor grade in primary adenocarcinoma of the prostate we analyzed 107 consecutive surgical specimens (78 radical prostatectomies and 29 transurethral resections). A hematoxylin-eosin-stained slide from a representative block of each tumor was examined, and primary and secondary Gleason scores were assigned in each case. Additional paraffin sections from the same block were stained immunohistochemically for p53 expression using the monoclonal antibody clone DO-1, a mouse IgG2a directed against a denaturation-resistant epitope of p53. Four of 54 (7.4%) low-grade tumors (combined Gleason score of 6 and below) and 11 of 53 (20.8%) high-grade tumors (combined Gleason score of 7 and above) revealed strong nuclear positivity for p53. When evaluated using only the primary Gleason score, none of 23 (0%) Gleason grade 2 tumors and 15 of 84 (17.9%) Gleason grade 3 or higher tumors were positive. These data demonstrate a positive association between p53 immunoreactivity and higher Gleason grade tumors (P = .04 for the combined score, P = .02 for primary score only). In addition, we noted occasional p53-positive nuclei in basal cells of benign glandular acini in regions flanking tumor. Focally positive nuclear staining also was demonstrated in basal cells from nine of 25 prostate glands exhibiting benign prostatic hyperplasia with no tumor. These results suggest that p53 overexpression might be associated with the known proliferative capacity of basal cells in benign hyperplastic prostate glands, and that mutations of p53 might play a role in the pathogenesis of a subset of high-grade prostate adenocarcinomas.