This article engages in detailed discussion of the material properties of water pipes made of polyethylene (PE). It describes the influence of properties of the material (including its geometric dimensions) on the level of reliability of pipelines made from PE 100. Values for the index of reliability obtained from analyses carried out using probabilistic methodswere compared with those recommended for the index in regard to a reference period of 50 years and different Reliability Classes (RCs). The fully probabilistic (3rd level) method – Monte Carlo simulation method was used to analyze the reliability. The probabilistic calculations were carried out with account taken of different values for the coefficient of variation describing material parameters, adopted as random variables; as well as the correlations between them. The work detailed here reports an influence of material geometry on the reliability index reported for the analysed pipeline made from PE. Where the analysed PE pipe was associated with a coefficient of variation for wall thickness at or over 0.07, this denoted non-compliance with standard PN-EN 1990:2002 as regards the minimum level of reliability recommended for a reference period of 50 years and Reliability Class RC2.