• An optic nerve swelling that was thought clinically to represent a lymphomatous infiltrate of the nerve head developed in a 51-year-old man with histiocytic lymphoma. Histologic examination of the eyes showed the presence of cytomegalovirus (CMV) in the nerve head, but no lymphoma cells. True lymphomatous invasion of the nerve head in systemic lymphoma is probably very rare. This case points out an unusual manifestation of adult CMV infection, and a new concern in patients with compromised immune systems.