To examine the effect of the major histocompatibility locus (HLA) and the duration of insulin-dependent diabetes (IDDM) on immune responses to insulin we assayed insulin induced proliferation of blood mononuclear cells and measured insulin antibodies in 66 patients with newly diagnosed and in 56 patients with longstanding IDDM matched for the age at onset (≤15 yrs). In up to two thirds of the patients blood mononuclear cells responded to insulins by proliferation, and insulin antibodies were found in two thirds of patients with IDDM of long duration. Insulin proliferation or antibodies were not associated to any particular HLA antigen. The frequency of HLA-DR3 in patients with newly diagosed IDDM was not increased unlike in patients with IDDM of long duration. In addition, HLA-B8 was associated to HLA-DR3 nearly twice as often in patients with newly diagnosed IDDM as in patients with longstanding IDDM. Thus, patients with IDDM of recent onset and diagnosed within the last three years more frequently responsed to insulin by proliferation and less often had HLA-DR3 than patients with IDDM of long duration and diagnosed about 20–25 years earlier.