Crack opening loads were measured local to the crack tip in aluminum alloy 7091 and 304 stainless steel using both optical and electron microscopy combined with the stereoimaging technique. These measurements showed that the crack peels open nonlinearly such that significant load increases are required to open the crack the last few microns from the tip. Based on measurements of the crack-tip opening load, the quantity U = ΔKeff/ΔK was found to depend on fatigue loading variables according to the simple relationship, U = 1 − Ko/Kmax = 1 − Ko(1−R)/ΔK, where Ko is a constant believed to be related to the threshold for pure Mode I fatigue crack growth. Although remote compliance measurements detect less crack closure than do the local stereoimaging measurements, these results follow a relationship similar in form to that above. Results are compared with previous measurements, as well as with analytical crack closure predictions based on the Dugdale-Barenblatt formulation. The latter underpredicts the experimental results and cannot predict their dependence on ΔK, thereby suggesting that either the model needs to be reformulated or other closure mechanisms are operative in addition to plasticity induced closure.