Objective To investigate the clinical significance of prostate specific antigen density (PSAD)value in the differential diagnosis from malignant and benign prostate disease .Methods 56 patients who enrolled in this hospital during April 2008 to No-vember 2012 were reviewed ,all of them were suspected as prostate carcinoma with free total PSA value at 4-10 ng/mL .Prostate volume were determined via transrectal ultrasound ,while serum PSA was determined by chemiluminescence method ,and all patients were finally pathologically diagnosed via tissue biopsy or prostatectomy .21 cases were proved to be prostate carcinoma (prostate carcinoma group) ,and the other 35 cases were proved to be benign prostate hyperplasia(BPH) .And the statistical significances of tPSA ,prostate volume and PSAD values were compared between the two groups .Results Compared with BPH group ,serum level of tPSA was insignificant ,while the values of prostate volume and PSAD were significantly lower in the BPH group than in prostate carciboma group .Considering the mean prostate volume(39 .27 mL) as the reference point ,40 mL was set as the cut-off value of prostate volume .Therefore ,there were 80 .95% prostate carcinoma group patients whose prostate volume were less than 40 mL , and 68 .57% benign prostate hyperplasia group patients whose prostate volume were larger than 40 mL .Considering the PSAD val-ue(0 .147) as the reference point ,0 .15 was set as the cut-off value of PSAD .Therefore ,the sensitivity ,specificity ,positive predic-tive value and negative predictive value among patients with prostate volume less than 40 mL ,the datas were respectively as 80 .95% ,72 .73% ,85% and 61 .54% .While ,among patients with prostate volume larger than 40 mL ,were respectively as 75% , 66 .67% ,27 .27% and 94 .11% .Conclusion PSAD was of certain value in the differential diagnosis of prostate carcinoma in PSA gray zone cases .If PSAD=0 .15 was consider as the cut-off point ,the sensitivity and specificity of differential diagnosis for prostate carcinoma decreased as the prostate volume increase .However ,the influencing factors are still to be elucidated .