Long-term behavior of naturally acquired anti-HBs antibody was tested every 6 months for 3 years in 22 dialysis patients. Fifteen of them maintained protective levels throughout follow-up (102 and 85.5 mUI/ml at the beginning and the end, respectively). Seven of them became anti-HBs and were submitted to a 40-μg booster injection of hepatitis B vaccine. Seroconversion was observed in 6 of 7 patients (85.7%) with a mean anti-HBs titer of 90.4 and 47.3 mUI/ml after 3 and 6 months, respectively. Protective anti-HBs level may be maintained longer in patients with natural immunity than in HBsAg-negative vaccinated subjects. Effectiveness of a reduced vaccination protocol in patients who have lost their natural immunization should be confirmed with further studies.