Molecular dynamics simulations have been used to investigate the effect of system size on the structure of model Langmuir monolayers. Several simulations with 64 molecules were performed in the range 0·185–0·40 nm² per molecule. In addition, results were obtained for 16 and 256 molecule systems at 0·23, 0·25 and 0·27 nm² per molecule. The simulations reveal a marked dependence on the number of molecules at 0·27 nm² per molecule. System size effects play an important role at intermediate densities, mainly due to the competition between the formation of a large tilt angle and the appearance of domains.