Blood matrix metalloproteinase 9 (MMP-9) level is a promising new diagnostic marker. The aim of the current case-control study is to investigate the correlation between the serum MMP-9 level with occurrence and progression of breast cancer. The serum MMP-9 level was investigated by gelatin zymography in 100 patients with breast cancer and 120 healthy participants. The average value of MMP-9 activity was significantly higher in patients with breast cancer than in control (P < .002), this value correlates with tumor stage (P = .005) and tumor size (P = .012). Our results suggest serum MMP-9 level is a potential diagnostic marker for predicting breast cancer occurrence and progression.