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Molecular dynamics simulations of water above a lipid (dimyristoylphosphatidylcholine) monolayer show that the structure and dynamics of the water are quite sensitive to the treatment of the long range forces. To obtain a quantitative estimate of the perturbations introduced into the water behavior by truncation of the potential, we calculate various structural and dynamical properties of the water for simulations employing different cutoffs. When potential cutoffs on the order of 10–14 A are employed, the properties of water near the monolayer exhibit perturbations of up to 20%–25% that extend 30–35 A into the water. Employing cutoffs on the order of 30 A leads to water behavior that is essentially uniform above the monolayer. We also present a quantitative description of the difference in the force experienced by individual atoms in simulations employing different cutoffs. Despite the significant difference in the water properties between simulations employing 14 A and effective 30 A cutoffs, the forces on the individual atoms differ by only ∼1%. It was also found that the motion of the lipids did not significantly affect the perturbations in the water properties. Simulations with shorter cutoffs in which the lipid atomic point charges were set to zero showed no perturbation in water structure. Water adjacent to a repulsive wall employed to maintain the system density also exhibited different behavior with effective 30 A cutoffs than with 14 A cutoffs.