In this preliminary study we analysed the impact of ovarian stimulations and the different protocols used for in vitro fertilizations (IVF) on the clinical activity of multiple sclerosis (MS). By matching the databases on MS and IVF of the past 10 years at the university hospital of Nantes, six patients have been found and, for five of them MS relapse rate seemed to be increased in the three-month period following IVF as compared to the previous three months and to two other control periods of three months (P<0.05, Friedman test). The increased relapse rate mainly concerned patients treated by GnRH agonists but not the patients treated by GnRH antagonists. This preliminary work suggests a possible impact of the treatments used for IVF on MS relapse rate. Further studies are now underway to validate these results on a larger scale, by including all cases reported in France.