It has long been standard practice to calculate the pressure in molecular simulations using the virial equation. However, in this Letter, we show that the virial equation does not apply when periodic boundary conditions are used. For pair-additive forcefields the problem is easily circumvented, but when non pair-additive forcefields are used, the virial equation should not be used for periodic systems. Examples of calculations for which errors could arise are simulations with polarizable forcefields or with ab initio molecular dynamics. Readers are warned not to use the virial equation in cases where it does not apply. (c) 2006 Elsevier B.V. All rights reserved.