This report describes five patients who, following a neurologically complicated primary Epstein-Barr virus infection, developed progressive or relapsing neurologic deficits. The sequelae in four patients followed 4 to 12 years led to the diagnosis of multiple sclerosis (MS). The fifth patient presented with acute disseminated sclerosis and exhibits diffuse neurologic deficits that have persisted for 2 years. We suggest that the diagnosis of an unexplained acute neurologic or psychiatric syndrome should raise the question of a primary EBV etiology. A precisely timed serologic and hematologic study of the blood is imperative to capture the essential evidence. The data presented represent a clinical association between a neurologically complicated primary EBV infection and both chronic and acute demyelinating disease. The evidence does not justify a conclusion that EBV virus causes MS.