The fatigue threshold ΔKTH, represents the nominal stress intensity range below which fatigue cracks are presumed not to propagate. Its magnitude, however, has been linked to the degree of crack closure. Since the extent of closure may vary, at fixed nominal ΔK, with such variables as crack length and loading history, the concept of a unique threshold as a material constant for a specific material/microstructure/environment system may become questionable. Of particular importance here for aerospace applications is whether crack growth remains dormant below ΔKTH in the presence of variable-amplitude loading, specifically during the occurrence of single compression cycles. In the present paper, a series of critical experiments on the effect of single compression overloads on the behavior of fatigue cracks arrested at ΔKTH is examined. Based on tests on I/M 7150-T6, 7150-T7, and 7475-T7351 aluminum alloys and 17-4 PH stainless steel, it was found that the occurrence of a single compressive cycle, of magnitude five times the maximum tensile load, caused immediate propagation of previously arrested threshold cracks at ΔKTH. Such observations are interpreted in terms of the degree of compressive residual stress in the cyclic plastic zone ahead of the crack and a measured reduction in crack closure following the compression cycle. The results serve to confirm the dependence of the threshold on crack closure and furthermore to illustrate the danger of utilizing nominal ΔKTH threshold values in damage-tolerant designs to predict fatigue life.