Nanocarbon materials have received considerable attention due to their unique structure and properties, which make them promising candidate materials for use in biomedical applications. In this review, we discuss the therapeutic applications of spherical nanocarbon materials, including fullerene nanoparticles, carbon nanohorn aggregates, nanodiamonds and porous carbon nanospheres, and their toxicology in biological systems. We put special emphasis on the antitumor effects of these multifunctional nanoparticles, which operate via novel mechanisms in a highly efficient manner. The low toxicities of these spherical nanocarbon materials as well as the possible effects of shape on toxicity are discussed.