Prostate-specific antigen (PSA) is the most important tumor marker for early detection and monitoring of prostate cancer (PCa) patients. PSA is also elevated in many patients with benign prostatic hyperplasia (BPH). The study of the serum PSA forms, free PSA (f-PSA) and PSA complexed with alpha 1-antichymotrypsin (PSA-ACT), may improve the discrimination between PCa and BPH. An immunoassay specific for f-PSA is reported with very low cross-reactivity (0.7%) to PSA-ACT. Serum specimens from BPH and PCa patients (determined by biopsy) with PSA levels from < 1 to > 100 ng/ml were tested. No f-PSA was detected in serum specimens from normal females (N = 50). Low levels (0-0.3 ng/ml) were detected in specimens from healthy males (N = 60). In specimens from PCa and BPH patients, the f-PSA to total PSA ratio (f/t) was found to range from 1% to higher than 60%. While maintaining an 80% sensitivity for cancer, the f/t ratio improved specificity to approximately 80%, as compared to 55% for total PSA alone. The receiver operating characteristics (ROC) curve analysis of the f/t ratio displayed a greater area under the plot (0.84) compared to total PSA alone (0.745). The results demonstrate that the f/t ratio significantly increases specificity for PCa detection compared to total PSA alone, showing the potential clinical value of the f-PSA immunoassay.