Extraprostatic extension is an unfavorable prognostic factor for prostate cancer. Consequently, it has been assigned to pT3a in the current 2002 tumor, lymph node, metastasis staging system. The aim of our study is to analyze the extent of extraprostatic extension by 8 quantitative methods and to determine which is best for substaging of pT3a tumors. We studied 83 patients with extraprostatic extension after radical prostatectomy for clinically localized prostatic adenocarcinoma. The extent of extraprostatic extension was evaluated using 8 quantitative methods. Univariate and multivariate analyses were performed to determine which measurement best predicts prostate-specific antigen (PSA) recurrence. In the univariate analysis, the radial distance of extraprostatic tumor measured by ocular micrometer was associated with PSA recurrence (P = 0.02). No significant association was observed between PSA recurrence and other measurements of extraprostatic extension, including focal versus established extraprostatic extension using Epstein's criterion, focal versus established extraprostatic extension using Wheeler's modified criterion, the number of extraprostatic neoplastic glands, unilateral versus bilateral involvement, circumferential length of extraprostatic tumor, unifocal versus multifocal involvement, and volume of extraprostatic tumor. In the multivariate analysis, radial distance remained an independent predictor of PSA recurrence (hazard ratio, 2.4; 95% confidence interval, 1.12-5.01; P=0.02). The radial distance of the extraprostatic extension measured by ocular micrometer is an independent prognostic factor for pT3 prostate cancer. Two-year and 4-year PSA recurrence-free survival was 62% and 35%, respectively, for those patients with radial distance <0.75 mm, as compared with 35% and 18%, respectively, for those with radial distance > or =0.75 mm. We recommend reporting this parameter routinely for radical prostatectomy specimens. The strength of its prognostic value for PSA recurrence makes it a potential criterion for incorporation into a future tumor, lymph node, metastasis staging system.