The objective of this report is to define the primary cause of fatigue cracks observed in the webs of welded plate girder test specimens. The average stresses in the web plate, which conformed reasonably well to those predicted by beam theory at the panel boundaries, are shown to have little connection with the occurrence of fatigue cracks, except in the tension region of girder panels under bending. The plate bending stress due to lateral deflection of the web is then examined and a method of estimating these stresses through use of measured web deflections is developed. By taking into account the torsional rigidities of the flanges and transverse stiffeners, a partial differential equation is established as the necessary condition to ensure equilibrium and compatibility between the web and its bounding elements. The subsequent solution of the equation is performed through the use of the finite difference technique. The accuracy of the resulting plate bending stresses is then examined. In comparison with stresses computed from measured strains, the method is proven reliable. It is shown that fatigue cracks always initiated at the points of maximum plate bending stress, that the sequence of crack formation on a single test girder is readily explained by these stresses and that a typical S-N diagram is obtained when the fatigue data of several girders are compared. It is concluded that the repetitive plate bending stress due to repeated lateral deflection of the web is a primary cause of fatigue cracks in thin-web welded plate girders.