A total of 137 patients with biopsy-proved anaplastic epidermoid nasopharyngeal carcinoma (NPC) seen at the National Taiwan University Hospital from December 1971 through December 1973, were studied serologically before radiotherapy and during the follow-up period up to December 1982. A clear control study on 134 healthy patients or patients with diseases other than NPC was also done. EB virus-associated anti-VCA antibodies in both IgG and IgA classes were titrated by means of indirect immunofluorescent antibody method on two to six samples of serum from the patients during the follow-up study. When seropositive standard was set at 1:640 for anti-VCA/IgG and 1:40 for anti-VCA/IgA, the seropositive rates were 63.5 per cent and 81.1 per cent for NPC patients before treatment and 2.9 per cent and 2.2 per cent for the control respectively. The differences of seropositive rates between the patients and control were statistically highly significant, as chi 2 greater than 111, p less than 0.0005. At the completion of radiotherapy and during the following year, some reduction and fluctuation of seropositive rates were seen in both cured and recurrent patients. From the second year after radiotherapy and thereafter, the seropositive rates were 72 per cent-100 per cent for patients with recurrences and 17.6 per cent-30.8 per cent for cured patients. The differences were significant (chi 2 greater than 24.8, p less than 0.0005). Therefore, high titres of anti-VCA antibodies may coexist with cancer tissue in NPC patients. IgA class antibodies is slightly higher in sensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)