OBJECTIVE: To explore the changes and significance of matrix metalloproteinases (MMP)-9 and tissue inhibitors of metalloproteinases (TIMP)-9 in serum of patients with coal workers' pneumoconiosis (CWP). METHODS: Serum MMP-9 and TIMP-9 were determined by sandwich ELISA in 188 cases of coal workers' pneumoconiosis (including 53 cases of coal pneumoconiosis, 67 cases of coal-silica pneumoconiosis and 68 cases of silicosis), 57 cases of 0+ miner (group 0+), 64 cases of healthy miner (dust exposed control) and 50 healthy people (healthy control) , and were further evaluated according to stage, complication, and the duration of exposure to dust in CWP group. RESULTS: The median of serum MMP-9 in coal pneumoconiosis, coal-silica pneumoconiosis and silicosis were 17.16, 15.14 and 17.50 ng/ml respectively. The levels of serum MMP-9 in silicosis and coal-silica pneumoconiosis were lower than that in dust exposed control and 0+ group (P < 0.05), and there were no differences among 3 groups of CWP. The levels of serum TIMP-9 in coal pneumoconiosis, coal-silica pneumoconiosis and silicosis were (330.00 +/- 108.42), (312.04 +/- 120.09) and (366.81 +/- 135.50) ng/ml respectively, and compared with dust exposed control and 0+ group, it increased in silicosis and coal pneumoconiosis significantly (P < 0.05). The levels of serum TIMP-9 in silicosis were higher significantly than that of coal-silica pneumoconiosis (P < 0.05). No significant difference was observed in the levels of serum MMP-9 and TIMP-9 in CWP according to CWP stage, complications and the duration of exposure to dust (P > 0.05). There was a positive correlation between age and the concentration of TIMP-9 in silicosis (r = 0.249, P < 0.05). A positive correlation was shown between the levels of serum MMP-9 and TIMP-9 only in dust exposed control (r = 0.294, P < 0.05). CONCLUSIONS: The levels of serum MMP-9 and TIMP-9 are associated with the harmful effect to miner resulted from dust. The apparent disturbance of MMP-9 and TIMP-9 might take part in the pathogenesis of CWP. Dynamic observation of its concentration in group 0+ and health miners might contribute to the prevention, early diagnosis and differential diagnosis of CWP.