Vaccination against oncogenic human papillomavirus (HPV) types is one key intervention for cervical cancer prevention. This follow-up study assessed the persistence of the systemic and mucosal immune responses together with the safety profile of the HPV-16/18 AS04-adjuvanted vaccine administered to young women aged 10-25 years. Serum and cervicovaginal secretion (CVS) samples were collected at prespecified time-points during the 48-month follow-up period. Anti-HPV-16/18 antibody levels in serum and CVS were measured by enzyme-linked immunosorbent assay (ELISA). At Month 48, all subjects remained seropositive for serum anti-HPV-16 and -18 antibodies. As previously observed, anti-HPV-16 and -18 antibodies levels (ELISA Units/mL) were higher in subjects vaccinated at the age of 10-14 years (2862.2 and 940.8) compared to subjects vaccinated at the age of 15-25 years (1186.2 and 469.8). Moreover, anti-HPV-16 and -18 antibodies in CVS were still detectable for subjects aged 15-25 years (84.1% and 69.7%, respectively). There was a strong correlation between serum and CVS anti-HPV-16 and -18 antibodies levels (correlation coefficients = 0.84 and 0.90 at Month 48, respectively) supporting the hypothesis of transudation or exudation of serum immunoglobulin G antibodies through the cervical epithelium. The HPV-16/18 AS04-adjuvanted vaccine had a clinically acceptable safety profile. In conclusion, this follow-up study shows that the HPV-16/18 AS04-adjuvanted vaccine administered to preteen/adolescents girls and young women induces long-term systemic and mucosal immune response and has a clinically acceptable safety profile up to 4 years after the first vaccine dose.