The stability of plane Poiseuille flow to periodic disturbances of finite amplitude was reinvestigated by the use of 55 harmonics instead of five in the Orr-Sommerfeld expansion, but all the (20) overtones in each harmonic were dropped. The fundamental mode was chosen as the one of lowest phase velocity c(r), so as to enhance the wall effects. The neutral curve obtained gives values of the critical amplitude lambda(c) that are considerably higher than in the previous calculation. Near the critical Reynolds number, the neutral curve approaches the values derived previously by an analytical method.