High-resolution B-mode ultrasonography enables quantitative measurement of the thickness of the intima-media layer of superficial large arteries noninvasively. We investigated the inter- and intra-observer variability of this measurement in the common carotid arteries in 10 randomly selected men. The maximal right and left carotid intima-media thickness (IMT) was measured with calipers during the scanning from frozen images by four observers in a blinded fashion. Three observers also repeated the scanning and the measurements twice with a week's intervals, with no knowledge of the previous readings. The inter-observer coefficient of variation (CV) was 10.5%. The intra-observer CV (mean of right and left CCA) was 5.4-5.8% for the three observers who carried out the measurements three times. The mean absolute difference between the first and third measurement was 0.087 mm. The intra-observer variation accounted for only 4% of the total measurement variability, 96% being attributable to inter-observer variation. These data show that most of the measurement variability in ultrasonographic B-mode IMT measurements is due to differences between observers, whereas the within-observer variability over time appears proportionately very small.