Background Occupational exposure to various neurotoxic chemicals has been shown to be associated with colour vision impairment. It seems that this can occur at low exposure levels, sometimes well below the recommended occupational threshold limits. This study was undertaken to determine the effect of exposure to petroleum derivatives (polyethylene, polystyrene) and solvents (perchloroethylene) on colour perception. Methods: Colour vision was assessed using the Ishihara plates, the D-15 test and the Farnsworth Munsell 100 Hue test. Two factories using petroleum derivatives and three dry cleaning premises were chosen at random. A total of 93 apparently healthy employees were recruited from the five workplaces. Two age-matched control groups comprising 56 people, who were support staff of the university with no exposure to petroleum, solvents or their derivatives, were also recruited. Results: All subjects passed the Ishihara test, showing that none had a congenital redgreen defect. Some of the exposed employees failed the D15 and had abnormally high F M l O O Hue scores. All control subjects passed all the colour vision tests. The D15 test showed that 28 per cent (26 of 93) of exposed employees had a colour vision defect whereas the FM 100 Hue test found that 63 per cent (59 of 93) had a colour vision defect. Most defects were of the blue-yellow type (22.6 per cent) when using the D15 test. However, with the FM 100 Hue test, most defects were of the nonpolar type with no specific axis (50.5 per cent). Mean total error scores calculated from the F M l O O Hue test for exposed employees were statistically significantly higher than those of the control subjects. Conclusion: Employees directly exposed to petroleum derivatives and solvents have a higher risk of acquiring colour vision defects compared to subjects who are not.