In 28 smoking and 37 non-smoking male metal workers from 7 small to medium sized plants the genotoxic effect caused by the occupational exposure measured as DNA single strand breaks in the peripheral mononuclear blood cells was investigated. Metal workers using synthetic cutting fluids are possibly exposed to the carcinogenic N-nitrosodiethanolamine (NDELA). In this investigation NDELA was detected in the air of the working halls in a concentration up to 1000 ng/m3 and in the cutting fluids in a concentration up to 135 mg/l (mean values per plant). Workers staying in rooms with a mean concentration of NDELA in the air of about 1 microgram/m3 revealed two times more DNA strand breaks than workers staying in an environment with less than 50 ng/m3 of NDELA (p < 0.01). Non-smoking workers with more than 4.5 h contact to cutting fluids per day showed an 1.5 times higher mean level of DNA strand breaks than their nonsmoking colleagues having had less than 4.5 h contact to cutting fluids (p < 0.02). Also workers having had work place-related complaints showed a statistically significantly higher level of DNA strand breaks compared to workers with no or no work place-related complaints. No significant correlation was obtained between the extent of DNA damage and the estimated extent of skin contact or the concentration of NDELA found in the cutting fluids. Therefore, for workers in this investigation NDELA incorporated by inhalation is probably more relevant for genotoxic damage than NDELA resorbed by skin. An increased level of DNA damage was found in metal workers depending on the concentration of NDELA in the air of the work places. However, without further investigations it cannot be excluded that also other concomitant agents in the environment were responsible for the observed genotoxic effect.