For 68 years latitude has been identified as an important risk factor in the occurrence of multiple sclerosis (MS), but not satisfactory explanation has been offered for this relationship. Epidemiological studies of MS, however, have failed to take into account the degree of change in the amount of ambient light over the course of the year, a variable which is closely related to photoperiod and latitude. Seasonal affective disorder (SAD), another illness for which latitude is a risk factor, appears to be related to the decrease in ambient light during the winter months, and offers some relevant insights into the geographical distribution of risk for developing MS. Researchers have found a relationship between degree of reported seasonal difficulties in a population sample and altered immunological function. Furthermore, the effects of bright light on mood have been shown to be regulated through the eye. We hypothesize that the risk of developing MS is related to impairment of the immune system caused by light deprivation prior to adulthood.