The Earth's magnetic (geomagnetic) field protects all living things on the planet from harmful effects of ioniz-ing and electromagnetic radiation and is suggested, in this research, to be related to global multiple sclerosis (MS) preva-lence. MS is a demyelinating disease of the central nervous system with an unknown aetiology. MS has a global, geograph-ic pattern of prevalence that shows high prevalence rates of the disease between 40 and 60 degrees north. Numerous ex-ogenous variables have been suggested to be factors in the expression of the disease but to date there is no single variable which best explains the pathological process. Excessive free radical formation appears to be a common factor in many of the previously correlated variables with MS. This study hypothesized that the vertical or horizontal geomagnetic field strength (or intensity) are more strongly correlated with global MS prevalence rates. Using data from available MS preva-lence studies (N=131) and online geomagnetic data for the field intensities, Pearson correlation and multiple regression analyses were performed. The results support the hypotheses with the strongest correlation being an inverse relationship between the horizontal field and global MS prevalence (r = -.607). The explanation for the inverse relationship with global MS prevalence and the horizontal component of the geomagnetic field is explained by protective effect from incoming cosmic radiation. This research suggests that living in areas of a weak horizontal field increases exposure to ionizing radia-tion and therefore increases the risk for developing MS. We propose a new scale of MS prevalence which captures the very high prevalence rates in some areas of the world.