The concurrence of multiple sclerosis (MS) and brain tumors has been reported, but it is not known whether MS patients are at greater risk of harbouring the latter. The most common cerebral neoplasms reported in MS patients were oligodendroglioma, astrocytoma, glioblastoma and gliomatosis. MS can also present as a mass lesion that mimics a brain tumor. To establish the correct diagnosis radiological follow-up and/or histological confirmation is needed. Two cases of coincidental MS and brain tumors are reviewed. One is a 26-year-old woman with relapsing-remitting MS and an anaplastic oligodendroglioma, the other a 49-year-old woman patient with relapsing-remitting MS and gliomatosis type 2. Both patients were treated with interferon-beta1b and both died from the tumor. The concurrence of MS and brain tumors could be purely coincidental, or the result of neoplastic transformation of reactive glial cells in the areas of demyelination. The combination of a brain tumor and MS, and interferon-beta treatment could also be pure coincidence or an unknown side effect of treatment. Although interferon-beta has been said to function as a tumor-suppressor protein, the influence of long-term treatment of MS patients on cancer development is not known.