All patients with congenital clotting disorders attending our centre routinely receive hepatitis B vaccination. We have assessed retrospectively the vaccine responses in 167 such individuals, with a follow-up period of up to 7.5 years. The initial postvaccine anti-HBs response was lower in older patients (p = 0.001), those infected with human immunodeficiency virus (HIV) (p = 0.05), and those who received intradermal rather than subcutaneous administration of vaccine (p = 0.08). It was estimated that the median time for anti-HBs levels to fall to 100 IU l-1 was between 36 and 42 months, with a shorter period for older or HIV-infected patients. Although the persistence of protective antibody levels can be predicted to some extent from the absolute levels of initial postvaccine anti-HBs, the wide variation in antibody decline between individuals precludes recommendation of the timing of vaccine booster doses solely based on this value.