An analytical model for axially loaded thin-walled stringer stiffened plates based on variational principles is exploited to study the sensitivity to initial geometric imperfections and the effects of altering geometric properties. Studies on different forms of global and local imperfections indicate that the post-buckling response governs the worst case imperfections. The investigation also focuses on the effect of changing the global and the local slendernesses on the post-buckling behaviour. The parametric space in which the stiffened plates are imperfection sensitive and susceptible to highly unstable cellular buckling is identified.