In February and in June 1998, two people developed acute hepatitis B following in-patient care in a district general hospital. Initial enquiries indicated their infections were not attributable to staff undertaking exposure-prone procedures (EPPs). We report the findings and implications of the subsequent investigation: a multi-disciplinary, multi-agency investigation, including molecular epidemiological analysis. Occupational Health records showed that staff involved in EPPs with the patients were HBsAg negative. No contact between the patients was identified nor were there failures in sterilization. The patients' HBV strains were identical, indicating a common source. A total of 231 out of 232 staff who might have treated either patient were tested for HBsAg; the remaining doctor, working abroad, was HBsAg- and HBeAg-positive and had the same HBV strain as the patients. On two occasions the doctor's hand had been cut while breaking glass vials, but there was no documentation linking these events to the two patients. The doctor had been vaccinated in 1993 and tested for anti-HBs prior to commencing work in 1997. The doctor was recalled to Occupational Health but did not attend and was not followed up. In total, 4948 patients potentially treated by the doctor received an explanatory letter and 3150 were tested for HBsAg. Only one was positive, and HBV sequencing showed no link to the doctor. Occasionally transmission of HBV from heath-care workers can occur in a non-EPP setting and the implications of this require examination by those setting national policy. Occupational Health Services should investigate clinical heath-care workers who do not respond to vaccination. They should ensure HBV carriers are identified and offer them appropriate advice to prevent transmission to patients.