BACKGROUND and PURPOSE: Environmental factors are thought to be important in multiple sclerosis (MS) pathophysiology. We aimed to evaluate if there was an association between MS relapses and some climatic factors in a Portuguese population. METHODS: Four year retrospective study analyzing 414 MS relapses in 249 consecutive relapsing-remitting patients. Non-parametric statistics were used to compare the distribution of relapses across months and seasons. Spearman's coefficient was determined to evaluate the correlation between relapses frequency and maximum and minimum atmospheric temperatures, humidity and atmospheric pressure. RESULTS: The mean number of relapses was not significantly different between months or seasons. No correlation was found between relapse frequency and any climatic factor. CONCLUSION: Our series is one of the largest addressing the influence of specific climatic factors on MS relapses. The number of clinical MS relapses seems to be unrelated to climatic factors.