In 1985-1986, 634 adult dialysis patients were tested prior to hepatitis B vaccination, representing 40% of the total patients in Latium, an Italian region. HBsAg and anti-HBs prevalences were 7.1 and 36.0%, respectively. Merck, Sharp & Dohme (MSD) and Pasteur plasma-derived vaccines were randomly allocated to the 44 dialysis units. An anti-HBs response greater than or equal to 10 IU/l was elicited in 58.5% of 236 subjects tested at 6 months, 63.4% MSD and 52.4% Pasteur (p n.s.). The relatively low response to the vaccine in this high-risk group stresses the need to improve the efficacy of the vaccination and to maintain policies of environmental control.