We report that a severe artifact appeared in molecular dynamics simulation of bulk water using the long cut-off length 18 angstrom. Our result shows that increasing the cut-off length does not always improve the simulation result. Moreover, the use of the long cut-off length can lead to a spurious result. It is suggested that the simulation of solvated biomolecules using such a long cut-off length, which has been often performed, may contain an unexpected artifact. (c) 2005 Elsevier B.V. All rights reserved.