Tumor volume has been considered an important variable in determining the probability of prostate-specific antigen (PSA) recurrence in prostatic adenocarcinoma. There have been many studies that have tried to determine an appropriate method of calculating tumor volume, but no single methodology has been agreed upon. We tested the hypothesis that the ratio of tumor positive tissue blocks to the total number of blocks submitted (positive-block ratio) can be used as an independent prognostic indicator for PSA recurrence. We analyzed 504 patients who underwent total radical retropubic prostatectomy between 1990 and 1998. None of the patients had preoperative radiation or androgen-deprivation therapy. Clinical records were reviewed. The mean positive-block ratio was 0.44 (median, 0.43; range, 0.05 to 1.0). The positive block-ratio was significantly associated with Gleason score, pathologic stage, surgical margin status, extraprostatic extension, seminal vesical invasion, lymph node metastasis, perineural invasion, and preoperative serum PSA level (all P<0.001). Using a multivariate Cox regression model, controlling for pathologic stage, Gleason score, lymph node metastasis, and surgical margin status, positive-block ratio was an independent predictor of PSA recurrence (hazard ratio, 2.3; 95% confidence interval, 1.06-4.83; P=0.03). Five-year PSA recurrence-free survival was 67% for those patients with positive-block ratio <or=0.43, as compared with 42% those with positive-block ratio >0.43 (P<0.001). Positive-block ratio is an independent predictor of PSA recurrence and this simple method of tumor measurement seems to be promising for quantifying tumor volume if our findings are validated by subsequent reports.