Objective To study the expression of kallikrein 7 (KLK7) in different prostate tissues and its clinical significance. Methods KLK7 mRNA levels in normal prostate epithelia (5 cases), benign prostat(ic) hyperplasia (BPH) epithelia (13 cases), prostate cancer and prostate cancer cell lines (8 cases) were analyzed by using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). Western blot was used to analyze the protein levels of human kallikrein 7 (hK7) in benign prostate epithelia and prostate cancer cell lines, hK7 expressions were examined in 20 normal prostate tissue specimens, 50 BPH specimens and 103 prostate cancer specimens by immunohistochemical staining.Results The mRNA levels of KLK7 in normal prostate, BPH and prostate cancer were 0.59, 0.52 and 0.02 respectively, mRNA levels of KLK7 were significantly different among the three groups (F=13.03, P＜0.01). mRNA levels of KLK7 were decreased in prostate cancers compared with that in benign hyperplastic prostate epithelial cells (P＜0.01) and in normal prostate epithelial cells (P＜0.01). No significant difference of KLK7 mRNA levels was found between normal prostate and BPH. The protein levels of KLK7 in normal prostate, BPH, DU145, LNCaP, PC3,22RV1 and BPH1 was 0.22, O. 40, 0.01, 0.05, 0, 0.03 and 0.14 respectively, hK7 protein level was down-regulated in prostate cancer cell lines compared to benign prostate epithelial cells. The expression of bK7 was observed in benign prostate epithelial cells, whereas little or no staining was observed in prostate cancer cells in immunohistochemical study, hK7 protein was detected in 13 of 20 (65%)normal prostate specimens, 38 of 50 (76%) BPH specimens and 18 of 103 (17.5%) prostate cancer specimens. The difference between the normal prostate and prostate cancer was significant (Z=-4.43, P＜0.01). The difference between BPH and prostate cancer was significant (Z=-7.77,P＜0.01) as well. However, no significant difference of hK7 protein level was found between normal prostate and BPH (Z=-1. 52, P＞0.05). Conclusions KLK7 expression level is down regulated in prostate cancer. KLK7 may play an important role in prostate cancer progression.