To evaluate the usability of biological monitoring of levels of exposure to N,N-dimethylformamide （DMF） vapor by urinary N-methylformamide（NMF）, we conducted a study in a factory in which DMF is used in the production of artificial leather. Severe liver damage in the worker exposed to 3 liters of DMF liquid was evidenced by raised levels of AST, ALT, γ-GTP and urinary NMF（327 IU/l, 158 IU/l, 233 IU/l and 854 mg/l, respectively）. A laparoscopy revealed the liver to be markedly deformed by extensive scarring. After this accident, we investigated the handling and management of DMF in this factory, and it revealed that, of workers engaged in the polymer coating section, 44% had over 40 mg/lof urinary NMF, and 32% had 10 to 40 mg/l, despite DMF concentrations in the air of less than 2.2 ppm. After changes of operational procedures by our intervention, the workers had over 40 mg/l and between 10 to 40 mg/l of urinary NMF decreased to 25% and 52%, respectively. Our occupational intervention revealed that urinary NMF concentration was a useful representative index of daily exposure to DMF in the factory.