Objectives: The study was performed to investigate the changes of urinary thiodiglycolic acid (TDGA) concentration in workers exposed to vinyl chloride monomer (VCM) according to the time of sampling urine. Methods: The personal exposure to airborne VCM was assessed and urinary TDGA concentration was sampled in 31 workers employed in a VCM and polyvinyl chloride (PVC) manufacturing factory. Urinary TDGA was sampled three times: before the start of the shift after 3 days off (TDGA1), after the end of the first-day shift (TDGA2) and before starting the following day shift after completing the oneday shift (TDGA3). Urinary TDGA in 30 workers who had not been exposed to airborne VCM was sampled after the end of the shift. A gas chromatography/pulsed flame photometric detector (GC/PFPD) was utilized to analyze TDGA concentration in urine after the urine was methylated with trimethylsilyldiazomethane (2.0M in diethyl ether). Results: The creatinine level was 0.179±0.271 mg/g in the control workers and 0.218±0.443 mg/g in the workers before the start of the shift after 3 days off (TDGA1), showing no significant difference (p=0.7035). Urine samples were compared according to sampling time in order to investigate the change of urinary TDGA concentration in the case of continuous exposure to airborne VCM. In VCM-exposed workers, urinary creatinine concentration was 0.434±0.623 mg/g in TDGA2 and 0.767±1.056 mg/g in TDGA3, which indicated a gradual but significant increase (p=0.024). In terms of the statistical correlation between airborne VCM and urinary TDGA to evaluate exposure dose per day, of the three urinary TDGA concentrations, TDGA3 showed the highest degree of regression (R 2 =0.4215) with 8h-TWA airborne VCM concentration. Conclusion: Based on this result, the excretion half-life of urinary TDGA was assumed to be less than 3 days, because the concentration of urinary TDGA at 3 days after exposure to airborne VCM was decreased to the level of urinary TDGA concentration in the control workers. The concentration of uri