Polyolefin blend miscibility is very important for a number of technological applications. Its experimental measurement is far from trivial and requires highly sophisticated expensive experimental techniques. In this work, the lattice-fluid theory of Sanchez and Lacombe is used to calculate the phase equilibria of binary polyolefin blends. The miscibility of high-density polyethylene with three branched polyolefins is computed as a function of polymer chain size and architecture, temperature, and pressure. The binary interaction parameter of the model is fitted to experimental data for the high-density polyethylene-poly(ethylene-alt-propylene) and used for all the blends examined. Lattice-fluid theory predictions are in qualitative agreement with limited experimental data available and with predictions from other theoretical models. The proposed relatively simple methodology can be used as a guideline to determine the phase behavior of polyolefin blends.