PURPOSE: To classify artifacts and to assess their frequency in magnetic resonance angiography of intracranial vessels using three-dimensional time-of-flight and phase-contrast techniques. MATERIALS AND METHODS: One hundred and eleven patients with suspected cerebrovascular disease were imaged on a 1.5T superconducting magnetic, resonance machine employing three-dimensional time-of-flight and phase-contrast magnetic resonance angiographic techniques. We retrospectively reviewed the artifacts in three-dimensional time-of-flight and phase-contrast magnetic resonance angiography of the intracranial circulatory system, comparing them with routine spin-echo magnetic resonance images and magnetic resonance angiography source images, and partially with conventional angiography. RESULTS: Artifacts in magnetic resonance angiography were classified as flow-related, and flow-unrelated, by patient, hardware, magnetic resonance angiography acquisition and postprocessing techniques. Type and frequency of flow-related artifacts included saturation artifact (100%), dephasing artifact(100%), phase-encoding ghost artifact (97%), turbulence artifact (14%) and flow displacement artifact (5%) on three-dimensional time-of-flight and phase-contrast magnetic resonance angiography, and phase aliasing artifact(2%) on three-dimensional phase-contrast magnetic resonance angiography. Type and frequency of flow-unrelated artifacts included stair-step artifact (100%) by three-dimensional reconstruction process, magnetic susceptibility artifact by carotid canal (69%) and metal (4%), maximum intensity projection artifact (30%) by maximum intensity projection algorithm, and motion artifact by respiration (20%) and voluntary movement(8%); these were seen on both time-of-flight and phase-contrast magnetic resonance angiography. Paramagnetic substance artifact by fat and paranasal sinus mucosa (100%), hematoma(14%) and gadolinium (5%) were seen on time-of-flight magnetic resonance angiography. CONCLUSION: In three-dimensional time-of-flight and phase-contrast magnetic resonance angiography, being familiar with the common artifacts and with their physical principles helps avoid misinterpretation of magnetic resonance angiography. An understanding of the causes of such artifacts will enable the radiologist to make rational changes in imaging technique and eliminate or reduce the effects of artifacts on magnetic resonance angiography.