A technique for measuring stress waves emitted during slow crack growth is briefly described. The number and size of the stress waves generated appears to bear a unique relationship to the amount of crack growth. From elasticity theory, a semi-empirical relationship is developed. Δ ≈ (Σg)2 E/K2, where ΔA is the incremental area swept out by the crack, Σg, is the sum of the stress-wave amplitudes associated with that increment of growth, E is the elastic modulus and K is the applied stress-intensity factor. This relationship is shown to be valid for about 100 data points obtained from crack growth in high-strength aluminum, titanium and steel alloys.