Second order effects of axial forces on displacements in compression members are small and can be neglected in a great many structures, which may then be designed for forces obtained by conventional first order theory. Extensive nonlinear analysis results are presented to document slenderness limits below which this is so for braced compression members, and major factors that affect lower slenderness limit predictions are investigated. A new lower slenderness limit formulation is presented for compression members braced against sidesway. The slenderness is defined in terms of a so-called normalised slenderness, that, in addition to the geometrical slenderness, is a function of axial force and reinforcement. The limit itself is a function of the first order end moment ratio. The formulation is rational and reliable and may replace, or used as an alternative to, existing lower limit formulations.