In the past, crack closure arguments have been successfully used to analyze the effect of large and small overloads on uniaxial fatigue life. The effective strain range, defined as the portion of a loading cycle during which the fatigue crack remains open, was found to correlate well with fatigue damage. Furthermore, an empirical function was found which described the dependence of crack opening stress on overload magnitude. These crack closure-based findings are utilized in this paper to examine recent multiaxial overload test results on a normalized SAE1045 steel. First, the von Mises' yield criterion combined with the classical flow rule is shown to properly describe the stress-strain behavior displayed by all proportional axial-torsion experiments, both constant and variable amplitude. Next, an effective equivalent strain-life relationship is found to unify overload fatigue data. A multiaxial crack opening stress model is then proposed which accurately calculates the cumulative effects from both in-plane and out-of-plane overloads in lowering the crack opening stress. This multiaxial crack opening stress model together with the established stress-strain curve and the effective strain-life curve are then shown to be capable of adequately calculating fatigue life for the large number of biaxial tests examined here.