Significance Low sunlight exposure and low vitamin D (vitD) levels are risk factors for the development of multiple sclerosis. However, there is still an ongoing debate, whether sunlight and vitD also modulate disease severity and worsening. Observational studies suggested vitD-dependent effects, but prospective supplementation studies have so far been inconclusive and reverse causality cannot be excluded as a source of bias. By using the sun-exposure measures vitD and latitude, we show correlations between vitD/latitude, vitD/disease severity, and latitude/disease severity in two multicentric cohorts. Although vitD cannot be proven as the causal factor, we provide evidence for clinically relevant effects of sunlight exposure. Furthermore, this study suggests sunlight-triggered pathways other than vitD could play additional and modulatory roles, as well.