The Gleason histologic score of prostatic adenocarcinoma in biopsy specimens (needle cores or transurethral chips) was compared with the Gleason score of corresponding radical prostatectomy specimens from 53 patients with localized prostatic carcinoma. The Gleason score assigned to the biopsy specimen was identical to that of the prostatectomy specimen in 51% of cases, was greater than that of the prostatectomy specimen in 4%, and was less than that of the prostatectomy specimen in 45%. The magnitude of discrepancy between the scores of the biopsy specimen and the prostatectomy specimen was directly related to the quantity of neoplastic tissue in the biopsy specimen. Discrepancies between the Gleason score of biopsy material and prostatectomy specimens were greater among biopsy specimens with low Gleason scores as compared with biopsy specimens with high Gleason scores. Given the small number of cases, these differences were not statistically significant. Clinical understaging of the primary tumor did not correlate with histologic undergrading of the prostatectomy specimen. It was concluded that prostatic biopsy should be repeated when the initial diagnosis of adenocarcinoma is based on only limited quantities of neoplastic tissue with a low Gleason score and management decisions may be influenced by the true Gleason score of the tumor.