A randomly selected subset of 100 pairs of baseline and 36-month follow-up carotid B-mode ultrasound examinations from the 919 patients participating in the Asymptomatic Carotid Artery Progression Study (ACAPS) were subjected to a blinded rereading at the conclusion of the trial to assess temporal bias in the measurement of carotid artery intimal-medial thickness (IMT). The original measurements of the primary outcome variable and five secondary outcome variables at baseline and 36 months, respectively, and the 3-year change in each of these variables, were compared with those obtained from the rereadings. For the primary outcome variable, the mean value of 12 IMT measurements obtained from predefined carotid segments, the mean difference (original-rereading) and the 95% confidence interval which resulted from the rereadings were -0.005 (-0.033, 0.023) mm at baseline and -0.009 (-0.031, 0.013) mm at 36 months. The difference in the 3-year change was -0.004 (-0.038, 0.028) mm. The 95% confidence interval for the mean difference between the rereadings and the original readings for the baseline and the 36-month follow-up examinations included zero for all of the six outcome variables as was also the case for the 3-year change in each variable. The magnitude of the mean differences for these 18 variables ranged from 0.004 to 0.034 mm. Intraclass correlation coefficients between the original readings and rereadings ranged from 0.56 to 0.87 with the 3-year changes in outcome variables tending to have lower correlations and the 36-month examinations higher correlations. The carotid IMT measurement process, when combined with uniform reader training, certification and monitoring of reading performance throughout the course of the study, can avoid the temporal bias observed in other studies.