Unbiased search by first-principles simulated annealing revealed irregular cage configurations for medium- sized B-n clusters (namely, boron fullerene) with n = 32-56, which are more stable than the previously proposed symmetric cages. The stability of these irregular cages can be understood by the three-center bonds as well as the polygonal holes on the cage surface. The delocalized distribution of molecular orbitals as well as the negative nucleus-independent chemical shifts (NICS) values for each boron cage indicates strong aromaticity. (C) 2010 Elsevier B. V. All rights reserved.