Abstract In this reply letter, we want to clarify the methods and some results of our former article “Multiple Chemical Sensitivity in chemical laboratory workers”. Background Multiple Chemical Sensitivity (MCS) is an acquired disease with an unknown biological mechanism. It is characterized by the development of sensitivity to certain chemical products. Most of the hypotheses formulated to explain the syndrome associate it to a previous exposition to some kind of volatile chemical. University researchers in chemical laboratories suffer a phenomenon of multi-exposition to chemical agents at low concentration during long periods of time although in an irregular form. Many of these chemical agents have similar properties to those suspicious of causing MCS. This article studies the prevalence of MCS in laboratory researchers. Methods The exposition to chemicals used by laboratory staff in our University departments was evaluated using the following methods: INRST ND 2233‐200‐05, from the French lnstitut National de Recherche et de Securite (INRS), and the NTP 937, by the Spanish Instituto Nacional de Seguridad y Salud en el Trabajo (INSST). In order to identify the persons affected by MCS we used the international reference questionnaire for MCS medical diagnosis, Environmental Exposure and Sensitivity Inventory (EESI), and specifically its shorter version, the Quick Environmental Exposure and Sensitivity Inventory (QEESI). Results In our study, we found no association between sex and prevalence of the MCS.