Although multiwalled carbon nanotubes show promise for delivering cancer therapy, there have been concerns raised over the potential of these nanotubes to cause cancer owing to their similarity in structure to asbestos. New research by Nagai et al. indicates that thin (approximately 50 nm in diameter) multiwalled carbon nanotubes with a crystalline-like structure can pierce the membrane of mesothelial cells and can lead to inflammation and the generation of mesotheliomas in vivo. Thick-walled (approximately 150 nm) or tangled (2–20 nm) multiwalled carbon nanotubes caused less damage and were less carcinogenic. Thus, controlling the diameter of multiwalled carbon nanotubes might reduce potential carcinogenic risk in humans.