An increased intima-media thickness of the common carotid artery is thought to be an early sign of atherosclerosis. Both B- and M-mode ultrasonographic techniques are used to measure the intima-media thickness of the common carotid artery (B-IMT and M-IMT, respectively). The present study compares intima-media thickness of the common carotid artery measured with the two techniques. Intima-media thickness was measured in a random population sample of 250 subjects. Comparison was made by mean and 95% confidence intervals of differences between B-IMT and M-IMT, by linear regression analysis, and by intraclass and concordance correlation coefficients. M-IMT was + 0.011 +/- 0.091 mm (95% confidence intervals: -0.167 to + 0.188 mm) larger than B-IMT, which was 0.661 +/- 0.136 mm (range: 0.380 to 1.120 mm). Intraclass and concordance correlation coefficients were 0.802 and 0.801, respectively. In conclusion, acceptable agreement exists between the two methods and there was no important systematic difference between B-IMT and M-IMT.