This article presents the study of a surgical block with serious deficiencies of Indoor Air Quality (IAQ) located in the Hospital del Rio Hortega (Valladolid, Spain). Block characteristics were identified and symptoms reports collected from 118 workers. At the end of the initial investigation, it was concluded that there was Sick Building Syndrome. Measurements of ventilation and contaminants were made of indoor air (CO, CO2, TVOC, anaesthetic gases, at six differents points simultaneously) and outdoor air (hospital incinerator). Studies with trace vapors (Xylene) were also performed on installations which crossed the surgical area to see whether a relationship existed between the different surgical areas studied and other areas of the health centre. Actions on contaminating foci, climatization-ventilation and personnel training were finally recommended to Administration to solve the problem.