We use two-dimensional mechanical models to investigate the effects of syntectonic sedimentation on fold-and-thrust belt development, testing variable syntectonic (wedge-top and foredeep) sediment thicknesses and flexural rigidities. Model results indicate a first-order control of syntectonic sedimentation on thrust-sheet length and thrust spacing. Thrust sheets are longer when syntectonic sediment thickness and/or flexural rigidity increase. Comparison with observations from several fold-and-thrust belts confirms this first-order control of syntectonic sedimentation.