Histologic grading of adenocarcinoma of the prostate gland is a reliable predictor of extension and metastasis. Studies involving correlation of grade between biopsy and prostatectomy specimens have traditionally involved biopsies using a large-bore (14-gauge) cutting needle. However, common practice has shifted to the use of biopsy cores with a smaller caliber (18 gauge). This study was undertaken to determine the degree of correlation of tumor grade between 18-gauge core biopsy samples and excised glands. Sixty-seven patients with stage A or B adenocarcinoma of the prostate gland who had previously undergone 18-gauge core biopsy, who underwent radical prostatectomies, were studied. The Gleason score was determined by referred consensus among three pathologists. There was exact agreement between biopsy and excision in 39 cases (58%), whereas 24 cases (36%) differed by one digit. Three cases (4.5%) were undergraded, and one case (1.5%) was overgraded by two or more points. Only six tumors (8.9%) would have been incorrectly specified by the degree of differentiation. Discrepancies in grade of two points or more were not more frequent in cases with a small tumor volume (< or = 10%) in the biopsy specimens. We concluded that with careful histologic evaluation, the grade of tumor identified in these smaller biopsy cores correlates well with that seen at prostatectomy.