Prostate cancer (PRCa) is one of the most common causes of cancer death in men and determinants of PRCa risk remain largely unidentified. Benign prostatic hyperplasia (BPH) is found in the majority of ageing men and has been linked with PRCa. CYP3A4 may influence PRCa through its role in testosterone metabolism. This nested case-control study assessed a CYP3A4 single nucleotide polymorphism as a risk factor for developing PRCa in patients with BPH. The CYP3A4 variant allele identified men with BPH who are at increased risk of progressing to PRCa (odds ratio 6.3, 95% CI 2.3-17.3), providing a potential tool to assist prediction strategies for this disease.