Retinoids have a variety of biological activities, including immunomodulatory action in a number of inflammatory and autoimmune conditions. Considering the pathogenesis of type 1 diabetes mellitus (T1D), in this study we examined the potential role for retinoids, etretinate and all-trans-retinoic acid (ATRA) in preclinical models of human T1D. When administered prophylactically to CBA/H mice made diabetic with multiple low doses of streptozotocin (MLD-STZ), both drugs effectively prevented clinical signs of diabetes. Prevention of T1D was associated with reduced emergence of primed (autoreactive) CD4⁺ CD25⁺ T cells, but not with the emergence of Foxp3⁺ Treg in the peripheral compartments. Moreover, the animals receiving ATRA exhibited reduced Th1/Th17 response and nitric oxide (NO) production in the peripheral lymphoid tissues, thus shifting the balance towards the anti-inflammatory cytokines. In NOD mice with spontaneous form of diabetes, ATRA prophylaxis, starting at a time point immediately before T1D onset, markedly reduced hyperglycemia and incidence of the disease. However, administration of ATRA to NOD mice in which the proportion and function of CD4⁺Foxp3⁺ Treg cells was abrogated by cyclophosphamide (CY), failed to permit progression to T1D. These findings suggest that effectiveness of T1D suppression by retinoids depends on the presence of Tregs which down-modulate immunoinflammatory events at the second “check-point” and allow diabetes progression.