The effects of specimen thickness and positive stress ratio on the fatigue crack growth behavior of Ti-6Al-4V are discussed in this paper. Fatigue crack growth rates are shown to increase with increasing stress ratio. Stress ratio is shown to have a strong effect on fatigue crack growth behavior. However, for the range of specimen thicknesses examined in this study, the fatigue crack growth rates are shown to not be strongly affected by specimen thickness. A single intrinsic fatigue crack growth rate curve is obtained when the fatigue crack growth rates are plotted against the effective (closure-corrected) stress intensity factor range. Multiparameter extensions to the Paris law are also presented for the combined assessment of the effects of specimen thickness, stress ratio, and crack closure on the fatigue crack growth behavior of Ti-6Al-4V.