Because background seroepidemiological data are required before nationwide vaccination against hepatitis B virus (HBV) is attempted, the rate of exposure to HBV was investigated in two high-risk Saudi groups: patients on haemodialysis and patients with congenital bleeding disorders. Although the HBsAg carrier rate was higher in patients with congenital bleeding disorders (11·1%) than in patients on haemodialysis (4·6%), the exposure rate to HBV was similar in both groups (38·8%–44·5%). Because of this high exposure rate it seems cost-effective, at least in the Saudi population or in countries which are endemic for HBV, to screen for HBV markers before vaccination against HBV is recommended.