Cross-sectional visualisation of the cellular and subcellular structures of human atherosclerosis in vivo is significant,as this disease is fundamentally caused by abnormal processes that occur at this scale in a depth-dependent manner.However,due to the inherent resolution-depth of focus tradeoff of conventional focusing optics,todays highestresolution intravascular imaging technique,namely,optical coherence tomography (OCT),is unable to provide crosssectional images at this resolution through a coronary catheter.Here,we introduce an intravascular imaging system and catheter based on few-mode interferometry,which overcomes the depth of focus limitation of conventional highnumerical-aperture objectives and enables three-dimensional cellular-resolution intravascular imaging in vivo by a submillimetre diameter,flexible catheter.Images of diseased cadaver human coronary arteries and living rabbit arteries were acquired with this device,showing clearly resolved cellular and subcellular structures within the artery wall,such as individual crystals,smooth muscle cells,and inflammatory cells.The capability of this technology to enable cellularresolution,cross-sectional intravascular imaging will make it possible to study and diagnose human coronary disease with much greater precision in the future.