In all kinds of analyses that reflect negative pore water pressures, the water retention curve (WRC) of the soil is considered to be one of the most important parameters. The objective of this paper was to illustrate what influence the choice of WRC has on the outcome of numerical simulation of negative pore water pressures. WRCs were estimated with three different methods for the same soil samples. It is evident that WRCs that are intended to describe the same material in most cases differ at substantial parts. Simple numerical analyses were also conducted to illustrate the effect these differences in estimated or determined WRCs have on the outcome of a numerical pore pressure analyses. Even minor variations in the WRC might have significant impact on the outcome of the pore pressure calculation. In the cases when negative pore water pressures are considered in pore pressure analyses, it is crucial that correctly determined, site specific water retention data is used.