The kallikrein family (KLK) has been implicated in cancer and may be useful as tumor markers. Here, we compared the 15 KLK genes' expression in malignant and normal breast tissues using real-time quantitative PCR. Most KLKs were expressed at lower levels in breast cancer compared to normal breast tissue. The only exception was the eightfold increase level of KLK4 in breast cancer tissues (P=0.008). KLK4 level was strongly associated with tumor grade (P=0.0015). Interestingly, based on laser cell microdissection analysis and immunochemistry, the up-regulation of kallikrein 4 occurred in the surrounding stromal cells. Our findings suggest that KLK4 may be associated with the development and progression of breast cancer and suggest its potential use in breast cancer monitoring.