Simulation results for a square-well (SW) fluid of range λ = 3 are presented and used to test the long-range (LR) theory recently developed for SW systems. The discontinuous molecular dynamics methods was used to obtain values of internal energies, pressures and orthobaric densities. The predictions of the second-order LR theory are in excellent agreement with the simulated results, showing that the LR approximation is highly accurate for ranges λ ⩾ 3. The accuracy of the van der Waals, mean-field andLRapproximations is established in terms of the temperature of the system and the range of the well.