Several creep theories are investigated for multiaxial loading of high-density polyethylene, a slightly cross-linked polymer demonstrating a nonlinear stress-strain-time relationship. Among the theories considered are nonlinear viscoelastic models, isochronous stress-strain diagrams employing a hyperbolic sine relationship, and multiple integral representations. The material parameters for these theories were determined from simple tension, compression, torsion and biaxial creep tests using single step loading. These theories were then analyzed for multiple step pressure-tension loading of a thin walled cylinder, in which the internal pressure and the axial load were varied independently of each other to check the effect of stress history on theory accuracy.