Objective: The impact of geomagnetic disturbances at the earth's surface is greatest at higher geomagnetic latitudes, particularly between 60-75°C. The prevalence of multiple sclerosis (MS) also has a latitudinal gradient. Therefore, the purpose of this study was to explore the hypothesis that geomagnetic disturbances may have a role in the development of multiple sclerosis (MS). Methods:A retrospective design was used to examine the correlations between the birth rate of MS patients and two geomagnetic indices, sunspots and the A-index. Birth rate data was derived from a clinical database of patients residing in a region near 60° N geomagnetic latitude from 1920-1980. Results:Results indicated that the intensity of geomagnetic disturbances, as measured by the A-index, had a stronger relationship to MS birth rate than the count of sunspots. Overall, the A-indices had small to moderate correlations with MS birth rates, combined (r=0.31-0.50) and by sex (male r=0.28-0.43; female r=0.24-0.50), over 60 years. Correlations were stronger as the length of exposure increased. That is, exposure in early childhood (birth year+3 years) and throughout childhood (birth year+12 years) exhibited generally higher correlations (early r=0.28-0.49; throughout r=0.24-0.50) than exposure during birth year alone (r=0.30-0.39). Conclusion:The results of this study indicate that geomagnetic disturbances may be a risk factor in the development of MS. The cumulative effect of exposure may be of importance. The advantages of examining exposure in utero and childhood are that birth cohorts can examine risk factors occurring early in life, can reduce the impact of period effects, and can meet the assumption of order-effect as we search for causal factors contributing to MS. The mechanism whereby geomagnetic disturbances may affect disease is unknown and warrants further exploration.