The influence of the target foil thickness on the characteristics of a proton beam produced by the interaction of a 1-ps laser pulse with a plastic foil target at intensities near 1017 W/cm2 has been investigated. It is shown that, for maximizing the energies and/or the current of forward-emitted protons, the target thickness must be properly selected. It should be smaller than the hot electron range in the target but greater than the characteristic path length of the electron heat wave generated by the prepulse and the leading edge of the laser pulse. If the target thickness is smaller than the heat wave path length, both the maximum and the mean proton energies can be a decreasing function of laser energy. © 2002 American Institute of Physics.