Multiple sclerosis is regarded as an autoimmune disease. The autoimmune process is thought to be triggered by early-life exposure to viral/bacterial antigens that share key peptide sequences with myelin protein (the target of autoimmune attack in multiple sclerosis). It has long been known that the incidence of multiple sclerosis is positively correlated with latitude, particularly in Caucasian populations. There is no agreed explanation for this latitude gradient, however. Ultraviolet radiation level is negatively correlated with latitude. Recent evidence suggests that ultraviolet-B is immunosuppressive, affecting particularly T-cell activity and delayed-type hypersensitivity. We hypothesize here that the latitude gradient of multiple sclerosis may reflect differential ultraviolet-induced suppression of autoimmune activity, particularly since the autoimmune profile of multiple sclerosis is characterized by disturbances of those T-cell-related activities that are specifically affected by ultraviolet-B. We propose some specific tests of this hypothesis.