We present here the first MS-case where rituximab-treatment led to grade IV neutropenia, with hospitalization and treatment of a serious infection with broad-spectrum antibiotics. The neutropenia resolved promptly with granulocyte-colony stimulating factor-treatment and the patient recovered well. Due to risk of recurring neutropenia rituximab-treatment was not re-administered. We discuss the mechanisms and occurrence of neutropenia as a side effect to rituximab-treatment of MS, and remind of the importance of monitoring rituximab-treated MS-patients for this rare but potentially dangerous side effect.