Melanoma in multiple sclerosis treated with natalizumab: causal association or coincidence?

We report a relapsing-remitting multiple sclerosis patient who received monthly intravenous natalizumab. After the fifth dose, the patient had a change in a long-standing mole. Ten months later, the mole became ulcerative and was ablated. Histological examination identified a spreading melanoma reaching the lower dermis (Clark level IV). Considering that at the moment the incidence of melanoma is estimable as about 5 per 100,000 multiple sclerosis person-years treated with natalizumab, and that, in the general population, the incidence of melanoma per 100,000 person-years is more than 10, we may speculate that the occurrence of melanoma during natalizumab treatment in multiple sclerosis is purely a coincidence.