For closely spaced, nanolithographically defined lines, a thin wall of resist remains to act as the metal line spacer. When exposed to a developer, and then rinsing solution, closely spaced resist walls may become unstable as a result of two effects: (1) internal stresses due to swelling, and (2) lateral surface forces between adjacent walls. In this article we perform a linear stability analysis of a thin polymer wall under the simultaneous action of internal stresses and lateral surface forces. We calculate a stability boundary, and show that internal stresses are necessary for the formation of deformation patterns of finite wavelength. We find that, for slightly subcritical swelling stresses a small lateral force can induce buckling, while, for slightly subcritical surface tractions large internal stresses are necessary to induce instability. The theoretical predictions are in good agreement with experimental data on poly(methyl‐methacrylate) walls produced by electron beam lithography.