The present paper deals with the stability properties of a planar channel flow driven by air injection through porous walls. Experimental investigations have been carried out in the so-called VECLA facility and a theoretical linear stability analysis has been performed. The nonparallel effects are studied by using three different stability approaches. They appear to be very significant for this particular flow. This study provides indeed an interesting example of an instability mechanism strongly related to the vertical component (usually negligible) of the mean flow. The obtained results finally agree very well with the measurements with respect to the amplified frequency range and to the streamwise amplification of the instability waves. ©1998 American Institute of Physics.