To determine the risk factors involved in myelodysplastic syndromes (MDS). A 1:2 case-control study was conducted in 20 Shanghai' hospitals over a 3-year period, covering 266 "de novo" MDS cases corresponded to FAB criteria, and 532 age- and gender-matched controls from same hospitals with MDS cases. Subjects were all surveyed using the same standard questionnaire including histories of medications (Chloramphenicol, Sulfonamides, Meprobamate, Phenytoin, Colchicine, Cyclophosphamide, Propylthiouracil, Anti-TB medication, Tolbutamide, Primaquine and Chinese traditional herbs such as Bezoar, Angelica, Arsenic, Thunder cloud vine) at least 5 years prior to the onset of the disease, tumors, exposure to benzene, heavy metal, organic phosphates, pesticides, petrol/diesel, organic solvents, dye and hair dye products, radiation, house decorating, alcohol and smoking. Occupational exposure to benzene increased significantly the risk of MDS (OR: 8.52, 95% CI: 2.30 - 31.10). Living near high voltage power lines (100 m) increased significantly the risk of MDS (OR: 1.60, 95% CI: 1.10 - 2.32). House decorating (one year prior to the onset of the disease) increased significantly the risk of MDS (OR: 2.40, 95% CI: 1.38 - 4.14). Other investigated occupational poisons did not increase significantly the risk of MDS. Hair dye products, alcohol and smoking did not increase significantly the risk of MDS. Occupational exposure to benzene, living near high voltage power lines and house decorating are the risk factors of MDS.