The prevalence of hepatitis B virus (HBV) infection in Cyprus was estimated in a sample of 795 blood donors, 388 armed forces recruits, 1872 hospital staff, 135 haemodialysis patients, 559 thalassaemic patients, 722 institutionalised adults, 98 mentally retarded children and 323 family contacts of HBsAg carriers. Commercially available radioimmunoassays were used for screening. Merck Sharpe and Dohme hepatitis B vaccine was used for a vaccination pilot program. Our results showed that the carrier rate of HBsAg in the blood donor and army recruit samples ranged between 0.77% and 1.01% and the prevalence of past infection between 11.1% and 13.6%. Among high risk groups the highest carrier rate was found in family contacts of HBsAg carriers (18.27%), in mentally retarded children (6.12%) and in institutionalized adult patients (5.40%). The frequency of immunes was found 28.5%, 12.2% and 33.2% respectively. The highest frequency of immunes was found in thalassaemic (82.9%) and haemodialysis (43.7%) patients. The carrier rate (2.94%) and the prevalence of immunes (22.5%) in hospital personnel was higher than in general population. Vaccination studies in a limited number of thalassaemic children and hospital personnel showed excellent acceptance of vaccination and excellent immunogenicity. We infer that HBV infection should be consider as a major public health problem in Cyprus. A National Vaccination program has to be implemented for containing its spread.