PURPOSE To determine the apparent diffusion coefficient (ADC) values of lipid and hemorrhage in atherosclerotic plaque in human carotid arteries in vivo and compare the values obtained from ex vivo carotid endarterectomy specimens. MATERIALS AND METHODS In vivo diffusion-weighted imaging (DWI) of carotid plaques was performed using a 2D single shot Interleaved Multislice Inner Volume Diffusion Weighted Echo Planar Imaging (2D ss-IMIV DWEPI) on 8 subjects who subsequently underwent carotid endarterectomy. A total of 32 slices used to construct the ADC maps were reviewed for the measurement of the mean ADC values in vessel wall, hemorrhage, and lipid necrotic core. The 8 endarterectomy specimens were scanned using by three-dimensional ms-IV-DWEPI. After the ADC maps were created, the mean ADC values in the same locations selected for in vivo values were calculated. RESULTS The mean ADC values obtained from in vivo DWI in normal vessel wall, lipid rich core, and hemorrhage were 1.27 ± 0.16, 0.38 ± 0.1, and 0.98 ± 0.25 × 10(-3) mm(2)/s, respectively. The mean ADC values in ex vivo lipid necrotic core, and hemorrhage were 0.33 ± 0.08, 1.28 ± 0.10 × 10(-3) mm(2)/s, respectively. These components mean ADC values obtained from in vivo and ex vivo ADC maps were compared. CONCLUSION ADC values of the carotid plaque components in vivo are consistent with values obtained from ex vivo endarterectomy specimens. The ability to obtain consistent plaque ADC values in vivo indicates that this technique could be an integral part of the basis for plaque component identification in conjunction with other MRI techniques.