The cytotoxic T-cell and humoral immune response to a commercially available influenza A-H1N1 subunit vaccine in 14 patients with type 1 diabetes mellitus was compared with the response in 13 healthy volunteers. Cytotoxic T-cell response to vaccination was poor in both patients and controls. At a calculated 50: 1 effector-target cell ratio, however, significantly more controls than patients showed an increase of over 5% cytotoxic T-cell mediated lysis after vaccination (P less than 0.05). In patients the cytotoxic T-cell response decreased with higher percentages of glycosylated haemoglobin (regression coefficient not equal to 0 with P less than 0.05). No significant difference was found between diabetic patients and control subjects with respect to antibody response after vaccination. Implications for vaccination strategy are discussed.