1,3-Dipolar cycloaddition reaction of azomethine ylides onto carbon nanohorns has been investigated by theoretical calculations based on the AM1, DFT and ONIOM methods. A preferential reactivity to cycloadditions at the conical-end of carbon nanohorns is found. The resulting unbalanced functionalization of carbon nanohorns is justified in terms of accumulated strain at sites near the conical-end, due to the presence of five 5-membered rings as compared to sites further removed from the tip, where only six-membered rings make up the nanohorn. (c) 2006 Elsevier B.V. All rights reserved.