The relationship between occupational exposure to methyl ethyl ketone (MEK) and its concentration in urine and blood was studied in a group of 72 workers in a printing factory. Personal exposure monitoring was carried out with passive samplers during the workshifts. The time weighted average (TWA) concentration of MEK ranged from 1.3 to 223.7 ppm, with a mean concentration of 47.6 ppm. In addition to MEK, toluene, xylene, isopropyl alcohol, and ethyl acetate were detected as the main contaminants in all samples. At the end of the workshift, urine samples were collected to determine the urinary MEK, hippuric acid (HA), and creatinine, and blood samples were also collected at the same time for determination of MEK. The concentrations of urinary MEK ranged from 0.20 to 8.08 mg/L with a mean of 1.19 mg/L and significantly correlated with TWA concentrations of MEK in the air with a correlation coefficient of 0.889 for uncorrected urine samples. The concentration of MEK in the blood was also significantly correlated with the TWA concentration of MEK with a correlation coefficient of 0.820. From these relationships, MEK concentrations in urine and blood corresponding to the threshold limit value-TWA (200 ppm; ACGIH 1992) were calculated to be 5.1 mg/L and 3.8 mg/L as a biological exposure index (BEI), respectively. Although the BEI for urinary MEK obtained from the present study was higher than that of previous reports and ACGIH's recommendation (2.0 mg/L), the BEI agreed well with a previous study in Japan.(ABSTRACT TRUNCATED AT 250 WORDS)