The antibody responses to influenza vaccination of a group of adult diabetic patients were compared with responses in a healthy group of regular volunteer vaccinees. The initial and final geometric mean hemagglutination-inhibiting antibody titers were lower in the patient group, but the relative increase in titers was greater for each of the vaccine components. The percentage of fourfold rises in individual titers was greater in the diabetic group than in the control group. It was concluded that patients with diabetes mellitus responded normally to influenza vaccination. This was confirmed in an additional study. There was no significant difference in the antibody responses of patients treated with insulin or oral antidiabetic agents. There was no impairment of diabetic control as a result of influenza vaccination when this was evaluated by measuring the concentration of glycosylated hemoglobin, or by random blood glucose estimations. There was no significant change in the serum insulin level after immunization in patients on oral diabetic agents. It was concluded that influenza vaccination was safe and effective in adult diabetic patients.