Preoperative serum prostate specific antigen correlates well with morphometrically determined prostate tumor volume in prostatectomy specimens. However, since prostate specific antigen is produced by hyperplastic as well as malignant prostatic epithelium, the contribution of hyperplastic epithelium (benign prostatic hyperplasia) to serum prostate specific antigen interferes with the ability of serum prostate specific antigen to predict tumor volume in individual patients. We wondered if the removal of benign prostatic hyperplasia tissue would increase the correlation between prostate specific antigen and tumor volume, and, thus, make prostate specific antigen a more accurate predictor of residual cancer volume after transurethral resection of the prostate. A total of 67 patients with clinical stage A cancer underwent radical retropubic prostatectomy (22, or 33%, with stage A1 and 45, or 67%, with stage A2 disease), and had pre-radical prostatectomy measurement of serum prostate specific antigen and morphometric determination of residual cancer volume in the radical prostatectomy specimen. The correlation between serum prostate specific antigen and residual cancer volume for all 67 patients was 0.66, and for stages A1 and A2 disease it was 0.64 and 0.70, respectively. All stage A1 cancer patients with a serum prostate specific antigen value of 1 ng./ml. or less had residual tumor volumes of less than 0.5 cc and all stage A cancer patients with a serum prostate specific antigen value of more than 10 ng./ml. had residual tumor volumes of greater than 0.5 cc. Of the patients 51% had levels of 1 to 10 ng./ml. and serum prostate specific antigen was not useful to predict residual tumor volume in this group. Serum prostate specific antigen measurements may be helpful in stage A1 cancer patients with levels of 1 ng./ml. or less, or greater than 10 ng./ml. in choosing the most appropriate therapy.