Paired sera taken from 63 subjects who developed cervical neoplasia in the course of a prospective study on this disease were examined for the presence of herpes simplex virus type 2 (HSV-2) antibody. The first serum was taken at enrollment into the study, while the second was obtained after development of the disease, i.e. 2-4 years later. Simultaneously, paired sera from a group of control subjects, who remained free of any cytological and colposcopical abnormalities throughout the study, were also investigated. Controls were matched with patients by age, age at first intercourse, number of sexual partners, smoking habits and history of diathermoelectrocoagulation of ectopic epithelium and transformation zone of the cervix. The first sera from these subjects were obtained at enrollment while the second ones were taken at the end of the study, i.e. 5 to 7 years later. Antibody titres were remarkably stable in both patients and control subjects. Seroconversions from HSV-2 negativity to HSV-2 positivity as measured by the increase in the II/I ratio or development of antibody reactive with HSV-2-specific glycoprotein G were rare and no significant differences between the patients and control subjects were detected. This indicates that the development of the disease was apparently not followed by immediate or early activation of latent HSV-2 infection.