Subjects diagnosed with high-grade prostatic intraepithelial neoplasia (HGPIN) at biopsy are at increased risk for developing prostate cancer (CaP). A prospective clinical trial was done to determine the safety and tolerability of a novel herbal amalgam, Zyflamend (New Chapter, Inc., Brattleboro, VT), with various dietary supplements in subjects with HGPIN. Men ages 40 to 75 years with HGPIN were eligible. Subjects were evaluated for 18 months. Every 3 months, standard blood chemistries and prostate-specific antigen (PSA) were monitored. Rebiopsy was done every 6 months. Tissue was evaluated for HGPIN or CaP and stained for cyclooxygenase-2, nuclear factor kappaB (NF-kappaB), interleukin-6, and thromboxane. Twenty-three subjects were evaluable. The median age was 64.1 years (range 46-75 years), and the mean (+/- SD) PSA level was 6.13 +/- 3.56 ng/mL. Side effects, when present, were mild and gastrointestinal in nature. There were no reported serious adverse events or toxicities. No significant changes in blood chemistries, testosterone, or cardiac function were noted. Forty-eight percent of subjects demonstrated a 25 to 50% decrease in PSA after 18 months. Of subjects who had the 18-month biopsy, 60% (9 of 15) had benign tissue, 26.7% (4 of 15) had HGPIN in one core, and 13.3% (2 of 15) had CaP at 18 months. A reduction in serum C-reactive protein was observed (95% confidence interval [CI] 0.7-1.7, p = .045). Immunoreactive staining demonstrated a reduction in NF-kappaB in the 18-month samples (95% CI 0.8-3.0, p = .017). Zyflamend alone and in combination with various dietary supplements is associated with minimal toxicity and no serious adverse events when administered orally for 18 months. Further studies are warranted to evaluate these agents in patients who are at risk for CaP.