In many forming processes, the mechanical properties of the workpiece material are deteriorated by the growth of voids during plastic deformation. In this study we present a new constitutive relation for void growth based on mechanical analyses of an isolated void growing in metallic media under various conditions of stress triaxiality. The incompressible, strain hardening, viscoplastic behavior of the matrix material is characterized by an isotropic state variable model. Systematic variations of the shear strength, mean stress, porosity and deviatoric deformation rate provided families of growth rate curves that were collapsed by appropriate scaling into two master curves. The forms of these curves and the scaling factors needed to obtain two master curves motivated the mathematical structure of the constitutive relation for void growth.