Macrophages and dendritic cells are important as antigen-presenting cells in the islet autoimmune response. We report decreased maturation of dendritic cells from blood monocytes of 61 patients with type 1 diabetes compared with 31 healthy controls (medians 26 and 35%, respectively, p = 0.0005). The dendritic cells also had reduced ability to cluster (96 and 124 clusters, respectively, p = 0.0005), and to stimulate autologous and allogeneic T cells. Because optimum antigen presentation is primarily required for tolerance induction rather than for immunisation, the defective maturation and function of diabetic dendritic cells might be the basis for disturbed activation of regulatory (suppressor) T cells.