The coincidence of multiple sclerosis (MS) and glioblastoma has been reported in several anecdotal reports. Little is known concerning the effects of radio- and/or chemotherapy on demyelinating brain lesions in MS patients. Moreover, there are no data concerning the effect of concomitant radiochemotherapy according to the STUPP protocol on the course ofMS in patients with coexisting glioblastoma. A 43-year-old male patient was diagnosed for relapsing-remitting MS in 1997. He received interferon and glatiramer acetate for immunomodulatory treatment and was stable until 2006 (EDSS < 1.5), when neurological deterioration occurred. He developed a left-sided hemiparesis, and an MRI showed right temporal contrast-enhancing mass lesion. A subsequent tumor resection was performed and histology revealed a glioblastoma. At the beginning of radiochemotherapy, treatment for multiple sclerosis (glatiramer acetate) was stopped. The tumor responded well to treatment and was clinically as well as radiologically stable until 9 months after diagnosis of glioblastoma. The typical radiological MS lesions remained unchanged. The patient died 12 months after diagnosis of glioblastoma due to tumor progression. This report demonstrates that concomitant radiochemotherapy according to the STUPP protocol, was safe in our patient with respect to the radiological as well as the clinical course of multiple sclerosis.