Between December 1979 and April 1982, 373 patients with untreated, undifferentiated carcinoma of the nasopharynx (NPC), 99 in Hong Kong, 120 in Tunis and 154 in Villejuif, entered a longitudinal study aimed at determining the clinical prognostic value of EBV serology after radiotherapy. A minimum of 3 years' follow-up was achieved for 319 patients (83 in Tunis, 95 in Hong Kong and 141 in Villejuif) who had regular clinical and serological testing at intervals of 6-8 months. No significant difference in initial serology (i.e., before any treatment) or variations of antibody titers at time of first follow-up was observed between patients who achieved complete remission after radiotherapy and those who did not. This included IgG and IgA antibodies to VCA, EA or EBNA. However, when patients with confirmed clinical remission 1 year after completion of radiotherapy were studied, the value of IgG/EA and mainly of IgA/EA increasing titers became highly significant for prediction of relapse, regardless of the initial titers. This demonstrated the clinical usefulness of EBV serology for NPC patients who have confirmed clinical remission after radiotherapy.