Twenty patients with well controlled Type 1 (insulin-dependent) diabetes of at least 10 years duration and 47 control subjects were vaccinated against the hepatitis B virus using the Hevac B vaccine. The vaccine was administered into the deltoid region on three occasions at intervals of 1 month. Thereafter a fourth dose was given to subjects still negative for antibody to hepatitis B surface antigen (HbsAb). The median rise of HbsAb titres was 230 mIU/ml in normal subjects and 50 mIU/ml in diabetic patients (p less than 0.001). Eight patients (40%) failed to reach HbsAb titres above 30 mIU/ml, the level considered to give optimal protection against the infection, whereas only one normal control subject failed to reach this level. Five patients (25%) showed no response despite a fourth dose of the vaccine. There was an increased frequency of HLA-DR7 in low responders and a decreased (less than 1.5) helper/suppressor lymphocyte ratio. Diabetic patients are thus less likely to mount a protective antibody response following vaccination against hepatitis. Since hepatitis B surface antigen is reported to be considerably more common in diabetic patients than control subjects, infection with hepatitis B virus may have a greater risk of chronicity in diabetes.