To determine the current risk of hepatitis B virus (HBV) infection in multiply transfused thalassemia patients, we tested sera from such patients in New York City for the hepatitis B surface antigen (HBsAg) and its antibody (anti-HBs) using radioimmunoassay techniques. Altogether 48 per cent of the patients had either HBsAg (4.5%) or anti-HBs (43.9%) positive sera. The prevalence of these HBV markers was related to both the number of units transfused and the year blood transfusion therapy was begun, although evidence suggested that the latter factor had the greatest influence. Donor HBsAg screening began in New York in 1969, and only one patient first transfused since that time had HBV marker positive serum. Thus, multiply-transfused thalassemia patients now appear to be at little risk of HBV infection from transfusions. Sera were also tested for antibody to the hepatitis A virus (anti-HA) using immune adherence hemaglutination. Anti-HA prevalence was only 4.9 per cent, no greater than rates reported among nontransfused children, providing evidence against a significant role for blood transfusions in hepatitis A virus transmission.