Human papillomavirus (HPV) transforming proteins E6 and E7 are uniformly expressed in HPV-associated cervical cancer. Our objective was to measure antibodies to HPV-16 E6 and E7 proteins in cervical cancer patients using an assay which would detect antibodies to conformational epitopes. Serum specimens obtained from two case-control studies of HPVs and cervical cancer were tested. The studies were performed in Cali, Colombia, South America and in 9 provinces of Spain. Cases consisted of women with invasive cervical cancer associated with HPV-16 or other HPV types and women with HPV-16-associated high-grade cervical intra-epithelial neoplasia (CIN-3). Controls for invasive cases and CIN-3 cases were women who had no cytologic abnormalities and who were matched for age and country of residence. Serum antibodies to HPV-16 E6 and E7 proteins were detected by radio-immunoprecipitation of in vitro translated proteins. Antibodies to the E6 and E7 protein were observed among 56% and 43%, respectively, of invasive cases and 1.7% and 4.1%, respectively, of controls. Antibodies to either protein were detected in 72% of sera from invasive cases and 5.8% of sera from controls. High antibody reactivity and antibodies to both proteins were found almost exclusively in invasive cases. The frequency of antibodies to the E6 protein and the E7 protein among CIN-3 cases did not differ significantly from the CIN-3 controls. Five women with HPV-18-associated invasive cervical cancer were negative for serum antibody to HPV-16 E6 and E7 proteins. Antibodies to HPV-16 E6 and E7 proteins appear to be partially virus-specific and disease state-specific markers of HPV-associated cervical cancer.