Fullerenes are a new carbonic allotrope having a cage structure. We investigated whether fullerenes containing one or two atoms of heavy metals could be an X-ray contrast material with little adverse effects. One or two atoms of dysprosium (Dy), erbium (Er), gadolinium (Gd), europium (Eu) and lutetium (Lu) were encapsulated into fullerene (C82), which was synthesized as a polyhydroxyl form (e.g., Gd@C82(OH)n, n=40, Gd - fullerenols). They were dissolved in water at maximum soluble concentrations and subjected to CT number analysis. The CT numbers of the solutions were measured using a 4- or 16-row multidetector CT scanner. The CT number of the water-soluble metallofullerenes were 56.0 HU for Dy@C82(OH)40, 111.5 HU for Er@C82(OH)40, 58.4 HU for Gd@C82(OH)40, 100.9 HU for Eu@C82(OH)40 and 23.3 HU for Lu2@C82(OH)40. The CT numbers of the metallofullerenes investigated in the present study were not high enough to be used in the clinic in place of iodinated contrast materials. However, if nanotechnology progresses in the near future, it may prove to have a possibility as an X-ray contrast material.