Concerns regarding rising water and sewerage prices have generated a renewed interest in measuring and assessing water utility performance. An efficiency analysis can serve as a basis for price setting decisions. This article examines the influence of ownership structure and corporate size on the efficiency of Estonian water companies, and assesses the empirical efficiency gaps through the lens of corporate governance and natural monopoly theories. To assess efficiency and the influence of ownership and corporate size on efficiency, we use a Data Envelopment Analysis and truncated regression with maximum likelihood estimation as well as an ANOVA test. The study sample consists of 43 water utilities, serving more than 68% of the Estonian population. One main finding of the study reveals that ownership structure does not affect the efficiency of Estonian water utilities, while efficiency does increase with corporate size: large water utilities outperform small utilities. An additional conclusion is that the Estonian water sector is too fragmented.