Objectives: This study examined the effect of shift work on developing the metabolic syndrome by comparing groups of exposed and unexposed Iranian drivers. Methods: We considered as night-shift drivers those drivers whose shifts included at least 15 h per week between 9:00 p.m. and 7:00 a.m. Daytime drivers were defi ned as drivers working regularly without shift work. 3039 shift work drivers were selected. These were matched with non-shift workers. The differences in baseline characteristics and the prevalence of the components of the metabolic syndrome were assessed with Student's t test, and chi-square tests. Results: We found central adiposity in 52.0% of the shift workers versus 42.6% of the day workers (p < 0.0001). The hypertension component was not signifi cantly related to shift work (p > 0.05); but there were signifi cant differences as regards other components of the metabolic syndrome (p < 0.0001). Among the shift workers, the odds ratios of the increased FBS, low HDL-C, higher TG levels, as well as higher waist circumference were 1.992 (95% CI: 1.697-2.337), 1.973 (95% CI: 1.759-2.213), 1.692 (95% CI: 1.527-1.874), and 1.460 (95% CI: 1.320-1.616), respectively. The metabolic syndrome was more common among the shift workers (OR = 1.495; 95% CI: 1.349-1.657). Conclusion: In evaluating such results, further consideration is needed to fi nd pathophysiological clarifi cation; in turn, stress linked to shift work must be considered to likely have had a relevant infl uence on the outcome. In our opinion, shift work acts as an occupational factor for the metabolic syndrome.