We demonstrate that oxidized single-wall carbon nanohorns (SWNHs), a type of single-wall nanotube, entrap cisplatin, an anticancer agent. We found that the cisplatin structure was maintained inside the SWNHs and that the cisplatin was slowly released from the SWNHs in aqueous environments. The released cisplatin was effective in terminating the growth of human lung-cancer cells, while the SWNHs themselves had no such effect. Cisplatin-incorporated oxidized SWNHs are thus a potential drug delivery system.