The difficulties of linking physiological changes in people with weather factors are great. They are increased manyfold when the study is limited to the macroclimate of the approximate geographic environs of a person or group of persons, since the physiological interactions are with the microclimate in immediate contact with the person, rather than with the geographic weather. The macroclimate is considerably modified by factors such as buildings, clothing, activity, etc. The influence of weather on multiple sclerosis has been the subject of frequent discussions. Many writers have expressed the thought that the disease is influenced by climate factors, and Ziegler1reported that exacerbations of the disease in Minnesota occurred most frequently in the spring. Limburg2was unable to find statistical evidence to support this contention but felt that his studies supported previous observations that the disease is more frequent in colder climates. He suggested, on the basis of