Infiltration and inflow of non-sewer water to wastewater systems (I/I water) poses a significant problem to health, economy, and the environment by affecting the capacity of the wastewater system. To determine effective measures to reduce the amount of I/I water, it is essential to identify its most important sources. In this thesis, the relationship between leaked drinking water and I/I water was investigated by using ordinary least squares (OLS) linear regression. The first question is whether there is a statistical relationship between the amount of drinking water leakages and the amount of I/I water in wastewater treatment plants in Swedish municipalities. Data from 207 Swedish municipalities were included in the analysis. The amount of unaccounted water, i.e. the difference between produced and charged drinking water, was used as a measure of water leakage. The second question treats the feasibility of statistically comparing municipalities to determine the importance of I/I water sources. The result of the OLS linear regression shows a statistically significant relationship between the level of unaccounted water and the level of I/I water in Swedish municipalities. This could be explained by a causal process where leaked drinking water infiltrates the wastewater system. Another explanation could be simultaneous problems in the water system and the wastewater system, potentially caused by the quality of the pipelines. However, caution should be exercised when drawing conclusions from statistical comparisons between water and wastewater flows in different municipalities. Variations in other aspects such as natural conditions and infrastructure, must also be considered.