During Mode III loading, the surfaces of a crack move parallel to each other; friction, abrasion, and mutual support of the contacting parts of the microscopically rough crack faces result. These energy-dissipative processes reduce the effective load at the crack tip, a phenomenon known as “sliding mode crack closure” (SMCC). This effect has been studied by performing crack growth experiments on circumferentially notched specimens of high-strength (AISI 4340) and mild steel (AISI C1018) under pure cyclic torsion and under combined loading (cyclic Mode III + static Mode I). Crack growth rates without the influence of sliding mode crack closure (“true” crack growth rates) can be determined using an extrapolation procedure. These crack growth rates are independent of crack depth, specimen diameter, and loading level and are therefore a material characteristic for Mode III fatigue crack growth. With the aid of the “true” crack growth curve the extent of sliding mode crack closure can be quantitatively determined and the changes in fracture mode explained. By superimposing a static tensile load the sliding mode crack closure is reduced whereas the “true” crack growth rates are only slightly influenced.