From 1978 to 1981, 151 patients with nasopharyngeal carcinoma (NPC) were enrolled in a prospective, collaborative study of North American patients, most of them white. Thirty-seven had World Health Organization (WHO) type 1 tumors, and 114 had WHO types 2 and 3 tumors. The anti-Epstein-Barr virus (EBV) profile of elevated antibody titers directed against viral capsid antigen and early antigen was seen in 85% of the patients with WHO types 2 and 3 tumors but in only 16% of the patients with WHO type 1 tumors. Geometric mean titers tended to be higher in higher stages of the disease in several staging systems. Low antibody-dependent cellular cytotoxicity at diagnosis appears to reflect a poorer prognosis, and the determination of antibody titers by this assay may prove to be useful for identifying persons in whom recurrent disease is likely to develop after conventional therapy. Anti-EBV titers can aid in diagnosis and treatment planning in patients with NPC, particularly those with occult primary NPC.