The aim of the study "Microbiological pollution as an indicator of well water quality in five counties of north-west Croatia" was to use microbiological analytics and precipitation data to determine whether microbiological pollution is an indicator of well water health in five counties of north-west Croatia in the period from 2012 to 2020. Microbiological analysis was carried out at the Veterinary Institute in Križevci, and the data on precipitation conditions in the observed period were given by the Croatian Meteorological and Hydrological Service (DHMZ). It was found out that there is a difference in the health quality of well waters between the monitored counties in the observed period and that precipitation conditions did not have a significant impact on the microbiological safety of water in wells. Microbiological analysis of the following parameters was performed: Escherichia coli, Enterococcus spp., Total coliforms, number of colonies at 22 ° C and 36 ° C and Pseudomonas aeruginosa. A total of 2,156 samples were analysed in all counties in 9 monitored years of which 545 (25.3%) were positive and 1,611 (74.7%) negative ones for microbiological parameters. In the observed period, dry, rainy, extremely rainy, and normal years alternated with respect to the average amounts of precipitation observed over multi-year averages. 2012, 2015, 2016, 2017, 2018, 2020 were classified as dry years, and 2013, 2014, 2019 as rainy years considering the multi-year average of the DHMZ.