In the past few years there has been an accumulating amount of evidence to suggest that the structural features of nanoparticles are responsible for dramatically different pharmacokinetic and body-excretion profiles. For example, a recent study convincingly illustrated that the mean hydrodynamic diameter of quantum dots (spherically shaped nanocrystals) is a determinant factor in achieving effective urinary excretion through the renal filtration barrier. In that study, however, the effect of nanoparticle shape on renal filtration was not examined at all. Our group has investigated the pharmacokinetic and excretion profiles of nonspherical, fibrilous carbon nanotubes (CNTs) and, in the present study, we have attempted to elucidate the mechanism by which such cylindrical nanoparticles can be excreted through the renal route. We have previously reported that surface-functionalized, water-dispersible, single-walled carbon nanotubes (SWNTs; average diameter 1 nm; average length 300–1000 nm) were capable of rapid and effective renal clearance and urinary excretion with a blood-circulation half-life of a few hours.