After a sharp increase in viral hepatitis cases, mostly type B, among the 2000 employees of a general hospital during three years, we conducted an investigation which consisted of obtaining data on employee cases and surveying many current employees. Of the 38 cases, 22 occurred in non-physician, ward employees. Of 189 current ward employees, 8% had antibody to hepatitis B surface antigen (anti-HBS) and 1% had hepatitis B surface antigen (HBSAg). Hepatitis B virus (HBV) seropositivity was highest for employees who worked closely with hemodialysis and renal transplant patients and for those who claimed that their ward was understaffed. Nine of the 38 cases occurred in clinical lab workers. Of 70 current lab employees, 17% were positive for anti-HBS and none for HBSAg. HBV seropositivity was highest for those working in the chemistry section (highest there among those performing blood-gas determinations and those working with the multi-channel autoanalyzers) and those who routinely got blood on their skin and clothes at work. All seropositive employees worked routinely with blood. These data support the hypotheses that many hospital employees contract hepatitis B from exposure to HBSAg-positive patients and many clinical laboratory employees contract it from exposure to HBV-contaminated blood.