In a preliminary study Bamezai and Kumar (1992) reported that a 24-h period of sleep deprivation may raise sister chromatid exchange (SCE) frequencies up to 193% in peripheral blood lymphocytes. This was reinvestigated to clarify the role of sleep duration as a confounder for SCE, which is a well-established parameter for biomonitoring in occupational medicine. In our study, the SCE baseline and the influence of a 24-h period of sleep deprivation (test period) on SCE were investigated for 20 non-smoking volunteers (10 females and 10 males; 20-29 years of age). There was no significant difference (Pall = 0.094) between the deviations of the two SCE rates of the control period (mean: -0.21 +/- 0.90 SCE) and the differences between SCE rates before and after sleep deprivation (mean: 0.42 +/- 0.94 SCE) of each proband. No significant difference was detected between females and males, and SCE did not correlate with age or sleep duration. Therefore we conclude that the influence of sleep deficit on SCE is in the range of a normal day-to-day variance, and has not to be taken into account when SCE is used for a genotoxic monitoring at the workplaces.