The reduced crack growth rate caused by each high-low load sequence of a complex load spectrum is modeled by a modified empirical constant-amplitude crack propagation relationship. The modification consists of the replacement of the crack rate term by a fractional derivative. The order of the derivative is a nondimensional parameter which is defined in terms of each high-low load sequence occurring in the spectrum. A cycle-by-cycle solution of the fractional differential equation yields the crack growth behavior for the spectrum. The approach is evaluated by comparing predictions with available experimental results for three aluminum alloys (7075-T6, 7050-T73, 2024-T3), two titanium alloys (Ti-6A1-4V, Ti-6Al-2Sn-4Zr-6Mo) and a nickel base superalloy IN-100. In all cases the method accurately predicted the general trends and estimates were well within a factor of two for 39 out of 40 tests analyzed.