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In 2000, since Hwang and Li proposed a remote user authentication scheme without password table using smart cards based on ID-based cryptosystems, the related works have been done. In 2002, Chien et al. proposed an efficient and practical remote user authentication scheme providing the many merits. However, Hsu showed that Chien et al.'s scheme is vulnerable to the parallel session attack. This paper proposes an improved scheme to overcome the weakness while maintaining the advantages of Chien et al.'s scheme.