We derive a measure of information leakage of quantum evolution from the information flow of open quantum systems. The measure has an information-theoretic interpretation in terms of classical-quantum correlations. It is then applied to analyzing information leakage taken place during interactions of individual systems via quantum algorithms, that is, quantum evolutions for computational tasks. Then, information leakage of individual systems as well as their information flow, entanglement generated during the evolution, and the computational speedups are compared.