['The purpose of this article is to evaluate whether the presence of perineural invasion (PNI) in the biopsy specimen is predictive of 5-year biochemical disease-free outcome after prostate brachytherapy.', 'Four hundred twenty-five patients underwent transperineal ultrasound-guided prostate brachytherapy using either 103Pd or 125I for clinical T1b/T3a NXMO (1997 American Joint Committee on Cancer) adenocarcinoma of the prostate gland from April 1995 to October 1999. No patient was lost to follow-up, and no patient underwent pathological lymph node staging. Two hundred twenty-one patients were implanted with 103Pd, and 204 patients were implanted with 125I. Of this cohort, 190 patients were implanted without supplemental beam radiation, and 235 received moderate-dose external-beam radiation therapy followed by a prostate brachytherapy boost. One hundred sixty-three patients received hormonal manipulation in conjunction with the brachytherapy implant (86 of these patients received moderate-dose external-beam radiation therapy before brachytherapy), and 262 patients were hormone naïve. Perineural invasion, defined as carcinoma tracking along or around a nerve within the perineural space, was identified in 105 patients (24.7% of the population). The median patient age was 68 years (range, 48-81 years). The mean follow-up was 37.1 +/- 15.2 months, and the median follow-up was 35.4 months (range, 6-74 months). Follow-up was calculated from the date of implantation. Biochemical disease-free survival was defined by the American Society of Therapeutic Radiation and Oncology (ASTRO) consensus definition.', 'When Kaplan-Meier survival analysis was performed, the presence of PNI did not predict failure. When PNI was entered into a Cox regression analysis with evaluated clinical predictors of failure (age, clinical T stage, pretreatment prostate-specific antigen level, and Gleason score) or treatment parameters (use of neoadjuvant hormonal therapy, supplemental external-beam radiation therapy, and choice of isotope), PNI did not add to the predictive strength of these variables. The median disease-free prostate-specific antigen level was 0.1 ng/mL for the entire cohort.'] Is perineural invasion predictive of biochemical outcome following prostate brachytherapy?