The purpose of this paper is to assess comparative efficiency in water and sanitation sector in Brazil. We run a Sthocastic Frontier Analysis model for a panel of 127 providers, covering more than 70% of urban population of the country. We search and find an explanation for cost efficiency, and we explore its interrelations with regions, property and independence of the providers. We use a database built on the SNIS (National System of Sanitation Statistics) to run a SFA cost model. We add to the formula, environmental and institutional variables, trying to capture differences in costs and efficiency attributable to the ambience of operation. We achieve a satisfactory model, showing a modest efficiency average, and we could patterns, even not well defined. The study has policy implications, in the discussion state-run versus municipal-run, public versus private, and corporitized versus dependent providers. The optimal industrial organization of the sector can be analyzed since the efficiency perspective