Twenty-nine percent of the health care workers who were vaccinated against hepatitis B showed no serologic evidence of hepatitis B immunity. It is unclear whether these subjects are nonresponders, lost immunity, or retained anamnestic potential. Booster vaccination response in 6 of 6 subjects suggests immunity. We recommend (1) postvaccination testing within 1 to 2 months to document immunity, (2) periodic anti-HBs monitoring, and (3) booster vaccination to maintain protective titer levels.