Toxicological and pharmacological studies suggest guidelines for the safe use of carbon nanotubes in medicine. The unique physical, chemical and electronic properties of carbon nanotubes (CNTs) have generated much interest in their potential medical applications. Although most studies have assessed the pharmacological efficacy, stability and toxicity of CNTs in vitro two recent reports, in the Journal of Toxicological Sciences and Nature Nanotechnology, explore their carcinogenic risk in vivo. Notably, these studies reveal that CNTs delivered to the abdominal cavity of mice can induce a response resembling that associated with exposure to certain asbestos fibers. What is the significance of these findings for efforts to develop CNTs as delivery vehicles for therapeutic and diagnostic agents? Carbon nanotubes are seamless cylindrical structures comprising single or multiple concentric graphene sheets.