In order to investigate a role of acetone in urine (AcU, mg/l) as an indicator of occupational exposure to isopropyl alcohol (IPA, ppm), AcU was measured in 80 male workers exposed to this substance in a plastic factory. The exposure concentration of solvent was also monitored personal diffusive sampling in the individuals during morning 4-hr shift. Urine samples were collected near the end of the shift and were analyzed for acetone by head-space gas chromatography. The correlation between airbornre concentration of IPA and its urinary metabolite acetone was significant: AcU (mg/l) = 0.031 x IPA (ppm) + 0.608, r = 0.75, n = 80, P < 0.001. We established 44 ppm as the lowest airborne concentration of IPA that caused excessive urinary excretion of acetone which could be discriminated from the endogenous production of acetone in non-exposed people. This concentration was as low as one ninth to one tenth of the current exposure limit of 400 ppm. At higher concentrations than 44 ppm, AcU was found to be a useful index for monitoring occupational exposure to IPA.