OBJECTIVES: To describe the initial assessment of early prostate cancer antigen (EPCA)-2 as a serum marker for the detection of prostate cancer and to examine its sensitivity and specificity. METHODS: Serum samples were obtained from 385 men: those with prostate-specific antigen (PSA) levels less than 2.5 ng/mL, PSA levels of 2.5 ng/mL or greater with negative biopsy findings, benign prostatic hyperplasia, organ-confined prostate cancer, non-organ-confined disease, and prostate cancer with PSA levels less than 2.5 ng/mL. In addition, a diverse group of controls was assessed with an enzyme-linked immunosorbent assay to detect an epitope of the EPCA-2 protein, EPCA-2.22. RESULTS: Using a cutoff of 30 ng/mL, the EPCA-2.22 assay had a 92% specificity (95% confidence interval 85% to 96%) for healthy men and men with benign prostatic hyperplasia and 94% sensitivity (95% confidence interval [CI] 93% to 99%) for overall prostate cancer. The specificity for PSA in these selected groups of patients was 65% (95% CI 55% to 75%). Additionally, EPCA-2.22 was highly accurate in differentiating between localized and extracapsular disease (area under the curve 0.89, 95% CI 0.82 to 0.97, P <0.0001) in contrast to PSA (area under the curve 0.62, 95% CI 0.50 to 0.75, P = 0.05). CONCLUSIONS: The results of our study have shown that EPCA-2 is a novel biomarker associated with prostate cancer that has high sensitivity and specificity and accurately differentiates between men with organ-confined and non-organ-confined disease.