Both blood viscosity and carbon monoxide (CO) has been associated with cardiovascular diseases (CVDs). In order to investigate the effects of chronic low-level CO exposure on the determinants of blood viscosity (hematocrit, plasma viscosity, erythrocyte deformability, and erythrocyte aggregation), 10 men exposed to CO at work for at least 6 months and 10 healthy controls were included in the study. Plasma viscosity was determined by a cone-plate viscometer, erythrocyte deformability and erythrocyte aggregation by laser-assisted optical rotational cell analyzer. Mean plasma viscosity of the group exposed to CO (1.4 +/- 0.1 mPa.sn) was significantly higher than that of the controls (1.2 +/- 0.06 mPa.sn) (p < .05). Plasma fibrinogen level of the CO group (275 +/- 11 mg/dL) was slightly higher than that of the controls (263 +/- 14 mg/dL). The rise in plasma viscosity by chronic low-level CO exposure may be the mechanism of CO-induced increase in the risk for CVDs.