Ten of sixty-one patients in a maintenance hemodialysis center seroconverted to hepatitis B surface antigen (HBsAg)-positive in August 1981. All but one were negative for antibody to hepatitis B core antigen, indicating early infection, and all received dialysis on the same days. Findings of case-control study showed that all "cases" received dialysis after the early morning shift, compared to 50% of controls (p = 0.01), and all cases used a multiple-dose vial of local anesthetic (bupivacaine), compared to 58% of controls (p = 0.03). At a common area used to prepare medications, an HBsAg carrier apparently stuck herself with a needle before drawing up bupivacaine, thus contaminating the vial that then served as the vehicle of transmission. Ten of eleven susceptible patients (those negative for antibody to HBsAg) who subsequently used bupivacaine and received dialysis seroconverted to HBsAg-positive, giving an attack rate of 91%. Serum samples from six of the ten cases were subtype Ad (or Adw), as was the implicated carrier's serum.