  We present a quantum dialogue protocol by using the
Greenberger-Horne-Zeilinger (GHZ) state. In this paper, we point out that the
`quantum dialogue' communication scheme recently introduced by Nguyen can be
eavesdropped on under an intercept-and-resend attack. We also give a revised
control mode to detect this attack. Hence, within the present version two users
can exchange their secret messages securely and simultaneously, and the
efficiency of information transmission can be successfully increased.
