  We show the flaws found in the customary fidelity-based definitions of
disturbance in quantum measurements and evolutions. We introduce the "entropic
disturbance" D and show that it adequately measures the degree of disturbance,
intended essentially as an irreversible change in the state of the system. We
also find that it complies with an information-disturbance tradeoff, namely the
mutual information between the eigenvalues of the initial state and the
measurement results is less than or equal to D.
