Artifacts associated with computed tomographic arterial tomography (CTAP) are well documented. We report a patient undergoing CTAP in whom laminar flow artifact occurred using Iopamidol 370 mg ml-1 but not when using Iopamidol 300 mg ml-1 under identical conditions. The denser Iopamidol 370 mg ml-1 mixed inadequately with unopacified blood in the portal vein. This was not a problem with the less dense Iopamidol 300 mg ml-1 which is likely to be a more reliable contrast agent for CTAP.