The compartment syndrome is uncommon in children. It has been seen following bony trauma and has also been reported in two patients following cardiac catheterisation. We report its occurrence in a 5-year-old child following femoral artery and femoral vein cannulation for cardiopulmonary bypass. We believe that the presence of a cannula in the femoral vein as well as in the artery was an important factor in impeding the circulation and thus causing the compartment syndrome. Prompt diagnosis and early fasciotomy saved the limb from amputation; however, a residual footdrop persists. The development of the compartment syndrome following femoral cannulation for cardiopulmonary bypass has not been reported previously.