The aim of this paper is to assess the eco-efficiency of dairy farms in Spain. To do so, we use data from a survey carried out in 2010 for the specific purpose of analysing the environmental performance of 50 dairy farms in the Spanish region of Asturias. The survey contains information on nutrients balances and greenhouse gas emissions which is used to calculate environmental pressure indicators. Eco-efficiency is measured using data envelopment analysis. We analyse the influence of farmers’ socio-economic characteristics and attitudes in explaining these eco-efficiency scores using truncated regression and bootstrapping procedures. On average, the dairy farms are found to be highly eco-inefficient. Among our results, farmers that are younger, that plan to continue in operation in the foreseeable future and that participate more in training schemes are found to be more eco-efficient. Self-reported positive environmental habits are also reflected in actual eco-efficient performance. We quantify these potential gains in eco-efficiency through a simulation analysis based on the estimated model’s coefficients.