OBJECTIVE To assess the clinical value of serum complexed prostate-specific antigen(cPSA) in the diagnosis of prostate cancer (PCa). METHODS Serum samples were obtained from 110 men with untreated benign prostatic hyperplasia (BPH) and 78 men with untreated PCa. The levels of cPSA, total PSA (tPSA) were determined by autoimmunochemistry luminescence method. RESULTS Both cPSA and cPSA/tPSA ratio were significantly different between patients with PCa and BPH (P < 0.005), especially, in men with tPSA values between 4.0-10.0 micrograms/L (the diagnostic gray zone). When cPSA/tPSA > or = 0.78 was taken as the cut-off value conjugated with tPSA < or = 10.0 micrograms/L, the sensitivity, specificity, negative predictive value and positive predictive value were as high as 97.8%, 95.8%, 81.9% and 96.5%. CONCLUSIONS With the introduction of cPSA and cPSA/tPSA ratio, early diagnosis of PCa by the assessment of tPSA has been made more sensitive and reliable, especially when the tPSA is within the diagnostic gray zone.