We examined the association between adulthood emotionality-activity-sociability temperament scale and preclinical atherosclerosis and, whether this association is mediated by cardiovascular risk factors (low-density lipoprotein cholesterol, systolic blood pressure, diastolic blood pressure and body-mass index (BMI)). The participants were a nationally representative sample of 537 men and 811 women from the Cardiovascular Risk in Young Finns study aged 15-30 years at the baseline in 1992 and aged 24-39 years at the follow-up in 2001. Carotid atherosclerosis was assessed by ultrasound scans of the common carotid artery intima-media thickness (IMT) and brachial flow-mediated dilation (FMD). In men, there was an association between the temperament dimension activity and IMT (ß = 0.08, p = 0.036) which was partially mediated by BMI (ß decreased from 0.08 to 0.05; p-value of Sobel test = 0.002). However, after correction for multiple comparisons the association between IMT and the temperament dimension activity in men was only of borderline significance. In women, there were no associations between temperament and IMT or FMD. These results suggest that a highly active temperament may contribute to early signs of atherosclerosis in men and that body mass may mediate this association.