Carotid intima-media thickness (IMT) can assess the cumulative effect of atherosclerotic risk factors and provides an independent predictor of future cardiovascular (CV) risk. The aim of this study was to investigate the progression of conventional risk factors in 933 long-term survivors from a Danish cohort with and without diabetes mellitus (DM) as predictors for attained carotid IMT during 35.6 (0.7) years of follow-up. Persons who participated in the first, the last and one of the intermediate rounds of the Copenhagen City Heart Study, and who had had an ultrasound-derived measure of the carotid IMT performed at the last examination were included in the analyses. The risk factors varied between persons with and without DM during the 36 years, but the difference in blood pressure disappeared in the fifth examination, where, in addition, total cholesterol was found to be lower in persons with DM. In this cohort there were no difference in attained carotid IMT between persons with and without DM at the last examination. The following risk factors were found to best predict carotid IMT: age, maximum systolic BP, average systolic BP, average BMI, minimum BMI, sex and years of smoking. The prediction of carotid IMT was clinically poor with a root mean-squared error of prediction (RMSEP) of 0.134 mm and a 95% prediction error probability interval of (−0.22; 0.30). Furthermore, the distribution of prediction errors was skewed to the right indicating that the prediction errors were larger among persons with high carotid IMT.