Human papillomavirus type 16 (HPV-16) can cause genital warts, cervical dysplasias and carcinoma of the cervix. Cell-mediated immunity is thought to be important in protection against the virus and in its elimination, but little is known about the mechanisms involved. In a cross-sectional study we have demonstrated proliferative T cell responses to peptides representing the HPV-16 L1 capsid protein (aa 199-409) in the peripheral blood of 63% of patients (n = 41) with histological evidence of cervical dysplasia and in 45% of healthy age-matched controls (n = 11). This was achieved by generating short-term T cell lines (STLs) from each individual in vitro against a beta-galactosidase-HPV- 16 L1 (aa 199-409) fusion protein for 2 weeks, and then identifying the HPV epitopes they recognized with overlapping synthetic peptides (15-mers) spanning this region in 3 day specificity assays. Histological grading and HPV typing by PCR were performed on patients' cervical biopsies taken at the same clinical visit as the peripheral blood samples. An immunogenic region was identified between aa 311-345 in 73% of patients (18% in controls) who responded to HPV-16 L1 (aa 199-409). The number of responders to this region was significantly higher in patients with HPV-16-positive biopsies when compared to those with HPV-16-negative biopsies (P = 0.006), as was the number of responders to individual peptides 311-325 (NLASSNYFPTPSGSM; p = 0.04) and 321-335 (PSGSMVTSDAQIFNK; P = 0.004) representing this region. The mean level of response to each individual peptide was also higher in the patient group than the controls (P < 0.05). The most significant finding was that all patients with evidence of a current HPV-16 infection responded to one or more L1 peptides (P = 0.0004) and 92% had high grade cervical intraepithelial neoplasia (CIN III). We also found that the CIN III group was more likely to respond to any L1 peptide than either the atypical group (P = 0.04) or the controls (P = 0.05). Data from four individuals showed that the majority of peptide-specific STLs were CD4+ but some CD8+ STLs were also detected.