We present a different attack on quantum key distribution based on the idea of adaptive absorption [J. Calsamiglia et al., Phys. Rev. A (64, 043814 (2001)]. The conditional beam-splitting attack is shown to be much more efficient than the conventional beam splitting attack, achieving a performance similar to the, powerful but currently unfeasible, photon number splitting attack. The implementation of the conditional beam-splitting attack, based solely on linear optical elements, is well within the reach of current technology.