Men with clinically localized prostate cancer are faced with a wide range of treatment options, and only Gleason grading is universally used as a histopathological prognostic factor for this disease. The significance of perineural invasion in diagnostic biopsies is controversial. Opinion about whether or not it should influence treatment decisions is currently almost equally divided. To address this, the authors performed a systematic review of studies that examine the association between perineural invasion and prostate cancer recurrence. MEDLINE, Embase, and the Web of Knowledge were searched from January 1990 to December 2005. Outcomes analyzed were the development of biochemical or clinical recurrence. Twenty-one articles on the association of perineural invasion in biopsies and prostate cancer recurrence after radical prostatectomy (n = 10) or radiotherapy (n = 11) were found but none on its significance in the context of watchful waiting. Structured data extraction was performed to allow comparisons between articles and to identify sources of heterogeneity to explain discrepancies in results. The considerable variation in study design, execution, and reporting precluded meta-analysis and quantitative risk estimation, but the weight of evidence suggested that perineural invasion in biopsies was a significant prognostic indicator, particularly in specific patient groups defined by presenting serum prostate-specific antigen levels and biopsy Gleason scores. Immediate treatment rather than watchful waiting may be more appropriate for patients with localized prostatic cancer and perineural invasion. However, the data are limited, and well-designed studies that use predefined stringent protocols are required to provide robust estimates of risk to aid in treatment planning. Cancer 2007. © 2006 American Cancer Society.