The molecular mass dependence of the zero shear-rate viscosity of low-density polyethylene (LDPE) melts is revisited for a series of LDPE samples of different molecular masses and densities. The long-chain branching structure and absolute weight average molecular masses are determined using a state-of-the-art size exclusion chromatography system coupled with multi-angle laser light-scattering. Creep experiments in a stress-controlled rotational rheometer are used to determine the zero shear-rate viscosity. The experimental results give evidence of an exponential molecular mass dependence of zero shear viscosity. It is demonstrated that these results can qualitatively be explained by comparison with recent theoretical predictions of the rheological properties of comb-branched model polymers.