Growing evidence suggests that environmental factors play a key role in the onset of multiple sclerosis (MS). This study was conducted to examine whether environmental factors may also be associated with the evolution of the disease. We collected data on smoking habits, sunlight exposure and diet (particularly consumption of vitamin D-rich foods) from a sample of 131 MS patients. We also measured their serum vitamin D concentration. The clinical impact of MS was quantified using the Multiple Sclerosis Severity Score (MSSS); MS was considered "severe" in patients with MSSS ≥ 6, and "mild" in patients with MSSS ≤ 1. The results showed a strong association between serum vitamin D concentration and both sunlight exposure (26.4 ± 11.9 ng/mL vs. 16.5 ± 12.1 ng/mL, p = 0.0004) and a fish-rich diet (23.5 ± 12.1 ng/mL vs. 16.1 ± 12.4 ng/mL, p = 0.005). Patients reporting frequent sunlight exposure had a lower MSSS (2.6 ± 2.4 h vs. 4.6 ± 2.6 h, p < 0.001). The mild MS patients reported much more frequent sunlight exposure (75% mild MS vs. 25% severe MS p = 0.004, Chi square test). A higher serum vitamin D concentration determined a lower risk of developing severe MS, adjusted for sunlight exposure (OR = 0.92 for one unit increase in vitamin D, 95% CI: 0.86-0.97, p = 0.005). A stronger inverse association emerged between frequent sunlight exposure and the risk of severe MS (OR = 0.26, 95% CI: 0.09-0.71, p = 0.009). Our data show that an appropriate diet and adequate expose to sunlight are associated with less aggressive MS.