The structure and stability of the (B12N12)(n) (n = 1, 2) cages have been computed at the B3LYP level of density functional theory. The most stable B12N12 cage is the T-h symmetrical structure with six isolated squares. The dimerization pattern of B12N12 differs strongly from those of C-60 and C-20. The most stable B24N24 structure, deduced from two B12N12 (T-h) cages, has an unexpected tube form.