Transient security assessment is an important part of power systems operation. In particular, on-line assessment is a present need in large power systems. This paper presents a feasible method based on pattern recognition techniques where all the on-line calculations for transient security assessment are replaced by the only evaluation of the sign of a security function dependent on measurements directly obtained from the power system. This security function has learning capacity to adapt to different network configurations and to new load and generation schedules, becoming, after sometime, independent of the power system model and of the methodology adopted in the off-line design.