In a 2-month period four cases of hepatitis B occurred in hospital staff. Three months later two patients, hospitalized for open-heart surgery when the staff members had been infected, developed acute hepatitis B. Sera from all six ill individuals were subtype ayw and e-determinant positive. Epidemiologic investigation showed that the four staff had been exposed 3 months earlier to an asymptomatic hepatitis B surface antigen-positive (HBsAg) patient who was also e positive. To determine transmission from staff to patients study of 17 open-heart surgery patients was undertaken. Four of 17 were either HBsAg or anti-HBs positive. No correlation between infection and contact with three of four ill staff members or receipt of blood products was noted; however, 22 (46%) of 48 arterial blood gas specimens had been obtained from infected patients by one staff member, an inhalation therapist; this compared with seven (4%) of 157 specimens she obtained from control subjects (P less than 0.001). Furthermore, she handled indwelling arterial cannulae 25 (76%) of 33 times in infected patients compared with three (3%) of 95 times in control subjects (P less than 0.001). Transmission may have occurred via the arterial cannulae from a severe exudative dermatitis on the therapist's hands.