The objective of this study was to quantitatively evaluate the environmental impact of tomato production in passive greenhouses and to identify the most relevant environmental issues. The LCA approach was used in this study. A tomato crop was cultivated in a steel-framed greenhouse; plants were grown in a closed-loop irrigation system in which the substrate was perlite bags. With the exception of the toxicity indicators, the main sources of environmental impact were fertilizer production and the process for manufacturing the materials used for the greenhouse structure and the auxiliary equipment, substrate, irrigation and recirculation equipment. The relatively short life span of plastic covered greenhouse structures and the minimal input of external energy involved in the production process are the most important factors related with the production process. The study compared the use of two cladding materials: rigid PC sheet with a lifespan of 10 years and flexible LDPE film with a life of three years. The types of cladding compared in this study (PC sheet and LDPE film) are not particularly relevant for environmental analysis. INTRODUCTION Life Cycle Assessment, LCA, is a tool for assessing the potential environmental impact of a given product or a system and considers the entire life cycle of a product from resource extraction to waste disposal. According to ISO standardisation guidelines an LCA study can be divided into four steps: goal and scope definition, inventory analysis, impact assessment, and interpretation. When defining goal and scope, the aim and the subject of the LCA study are determined and a ‘functional unit’ for instance, yield per square meteris defined. In inventory analysis, all extractions of resources and emissions of substances attributable to the studied functional unit are listed. Impact assessment determines the magnitude of the potential impact of individual substances within each impact category. Impact categories correspond to particular environmental problems, such as human toxicity or ozone depletion. The final step in an LCA study involves interpreting the results obtained from the previous three steps, drawing conclusions and formulating recommendations (Antón et al., 2002). When this procedure is applied to an agricultural product grown in a greenhouse, not only are the adverse environmental effects derived from the whole of the production process in question (these include: eutrophication, pollution associated with the use of pesticides, and waste generation) but other aspects must also be taken into account, such as the environmental impact associated with the manufacturing and transport of raw materials, energy use, the production of materials used for building greenhouses, the waste generated when the facilities are retired from use, and all of the other aspects that form part of the life cycle of the considered product and which may produce a negative environmental impact. All of them will have to be accounted for, by attributing a different environmental impact to each product unit (Antón, 2004). Previous works on the application of LCA in protected horticulture relate to projects in northern Europe. They show that reducing heating requirements is a priority if environmental loading is to be limited (Jolliet, 1993, Bucher et al., 1996, Nienhuis et al., Proc. IC on Greensys Eds.: G. van Straten et al. Acta Hort. 691, ISHS 2005 49