We perform first-principles calculations to study the relative stability of boron nitride fullerenes BnNm. We consider fullerenes with octahedral, icosahedral and tubular shapes, in a size range with (n + m) between 20 and 288. A scaling analysis allows the total energy comparisons to extend to larger sizes. We find that tubular-shaped fullerenes are as stable as octahedral-shaped ones for (n + m) <= 80. We also find that, in N-rich conditions, icosahedral fullerenes with line defects composed of N-N bonds may be as stable as octahedral fullerenes. (c) 2006 Elsevier B.V. All rights reserved.