We assessed the persistence of anti-HBs antibody and immune memory in a cohort of 571 teenagers vaccinated against hepatitis B as infants, 17 years earlier. Vaccinees were followed-up in 2003 and in 2010, i.e. 10 years and 17 years after primary vaccination, respectively. When tested in 2003, 199 vaccinees (group A) had anti-HBs <10 mIU/ml and were boosted, 372 (group B) were not boosted because they had anti-HBs ≥10 mIU/ml (n=344) or refused booster (n=28) despite anti-HBs <10 mIU/ml. In 2010, 72.9% (416/571) participants had anti-HBs ≥10 mIU/ml (67.3% in group A vs 75.8% in group B; p=0.03).The geometric mean concentrations (GMCs) were similar in both groups. Between 2003 and 2010, anti-HBs concentrations in previously boosted individuals markedly declined with GMC dropping from 486 to 27.7 mIU/ml (p<0.001). Fifteen vaccinees showed a markedly increase of antibody possibly due to natural booster. In 2010, 96 individuals (37 of group A and 59 of group B) with anti-HBs <10 mIU/ml were boosted; all vaccinees of the former group and all but two of the latter had an anamnestic response. Post-booster GMC was higher in group B (895.6 vs 492.2 mIU/ml: p=0.039). This finding shows that the immune memory for HBsAg persists beyond the time at which anti-HBs disappears, conferring long-term protection. This article is protected by copyright. All rights reserved.