The biological applications of carbon nanotubes are very promising. Much progress has been made in using this novel material for molecular delivery in vitro. The current work of Liu and colleagues demonstrates the first carbon nanotube drug delivery for in vivo cancer treatment with animal models. Paclitaxel (PTX), a commonly used chemotherapy drug, is conjugated to short, single-walled carbon nanotubes (SWNT) functionalized by branched polyethylene glycol (PEG) via a cleavable ester bond, affording a water soluble, biocompatible SWNT-PTX conjugate (bottom panel). Intravenously injected SWNT-PTX exhibits higher efficacy in suppressing tumor growth than clinical Taxol® in a murine 4T1 breast-cancer model, owing to prolonged blood circulation and 10-fold higher tumor PTX uptake by SWNT delivery through the enhanced permeability and retention (EPR) effect. The cover figure upper panels compare the apoptosis levels of tumor cells after receiving various treatments including Taxol® and SWN-PTX. Higher apoptosis level and reduced cell proliferation is seen in the SWNT-PTX case. Drug molecules carried into the reticuloendothelial system are released from SWNTs and excreted via biliary pathway without causing obvious toxic effects to normal organs. Without attached drugs, PEG functionalized SWNTs are nontoxic to large numbers of injected mice. Thus, nanotube drug delivery is promising for high treatment efficacy and minimum side effects for future cancer therapy using low drug doses. Further work will focus on the optimization of SWNT surface chemistry and incorporation of targeting ligands for nanotube-based drug delivery systems. Much improved treatment efficacy and further reduced side effects are expected for a wide range of chemotherapy drugs. For details, see the article by Liu et al. on page 6652 of this issue.