The laminar flow in an axisymmetric sudden expansion has been studied experimentally. Using a nominal diameter ratio of 0.5 and well‐defined inlet velocity profile conditions, data are presented that, for the first time, demonstrate clearly that the onset of shear layer instabilities is dependent on the inlet velocity profile. The reattachment length of the shear layer (normalized by the expansion step height) as a function of the inlet Reynolds number is linear with slopes of 0.096 and 0.068 for upstream velocity profile conditions of Poiseuille and nominal plug flow, respectively.