The aim of this paper is to research into the interactions of factors influencing the frequency of pipeline failures and to determine their impacts. The research was carried out on a representative part of the water supply system of Zagreb, the capital of Croatia, i.e. on a District Metered Area (DMA), using the log-linear analysis of frequency tables. The first influential factor that was analyzed was the condition of pressure regulation. The DMA was observed in the period when the system was not in the condition of pressure regulation (January 2005–March 2006) and in the period when the system was in the condition of pressure regulation (March 2006–September 2008). Other influential factors that were included into the analysis were the diameter, the material and the age of pipes. In addition to the main goal established in this general way, a specific goal related to the condition of regulation in interaction with remaining influential factors was also set. Using the log-linear analysis, general results about the influence of the interaction between observed factors were obtained. The specific goal was reached by conclusions made about the effect of the relation between the DMA condition of regulation and remaining influential factors, i.e. diameter, material and age of pipes on the frequency of pipeline failures.