Linear stability of wall-bounded shear layers modified by distributed suction has been considered. Wall suction was introduced in order to simulate distributed surface roughness. In all cases studied, i.e. Poiseuille and Couette flows and Blasius boundary layer, wall suction was able to induce a new type of instability characterized by the appearance of streamwise vortices. Results of calculations show that a linear model of suction-induced flow modifications provides a sufficiently accurate representation of the basic state. The effects of an arbitrary suction distribution can, therefore, be assessed by decomposing this distribution into Fourier series and carrying out stability analysis on a mode-by-mode basis, i.e. once and for ever.