A randomized study of injecting drug users in a Danish prison comparing vaccination at 0, 1 and 3 weeks with the 0, 1 and 6 months schedule (20microg Engerix B i.m.) was conducted. Due to a low participation rate, a second nonrandomized study was conducted in Estonia where all prisoners were vaccinated with the short schedule. In the randomized study, the compliance with three doses was 63% (12/19) in the 3-week schedule compared to 20% (3/15) in the 6-month schedule (P = 0.017). In the nonrandomized study, the compliance was 81% (457/566) and the seroprotection rate at month 7 was 67% (97/145), resulting in protection of 54% of the population at risk. This was significantly higher than the estimated 34-42% protection that would have been achieved with the 6-month schedule.