There is an overall increase in the worldwide prevalence and incidence of multiple sclerosis (MS). Studies from several countries also demonstrated an increase of female/male ratio over time denoting an increase in the incidence of MS particularly in women. In this study we sought to assess the trends in MS incidence and prevalence in males and females over recent decades in Isfahan, Iran, which differs from other regions in terms of environmental and lifestyle changes. We determined female/male ratio by year of birth (YOB) in 1584 patients with MS registered with Isfahan Multiple Sclerosis Society (IMSS) from April 2003 to August 2007. A comparison of sex ratio of MS patients by YOB showed a significant, progressive, gradual increase, with an apparent interruption in the late 1960s. In this study year of birth is a significant predictor for sex ratio (p < 0.001, χ² = 17.130, Spearman’s rank correlation r = 0.893). Our findings show that there is a significant increase in the incidence of MS among females for the the last decades in the Isfahan province of Iran. This rapid increase may be related to changes in environmental interactions rather than genetic factors, and among them vitamin D insufficiency, enhanced diagnosis, and lifestyle changes appear to be more plausible causative factors.