Type 1 diabetes is caused by the immune-mediated destruction of pancreatic beta cells. Animal models of the disease demonstrate an increased susceptibility of beta cells to immunological attacks due to their defective stress-responsiveness. To investigate the stress-responsiveness in human type 1 diabetes we analyzed the heat-inducibility of the dominant stress protein heat shock protein (Hsp)70 in diabetic patients at different disease stages. At diabetes-manifestation heat-induced Hsp70 levels in peripheral blood mononuclear cells (PBMC) reached only about 25% of the levels expressed by heat-treated PBMC from non-diabetic subjects (p<0.05). Heat-responsiveness improved with disease duration and was re-established at more than eight months after disease-manifestation. Hyperthermia-induced Hsp70 expression was decreased by the T-helper 1-associated cytokine interferon-gamma and increased by the T-helper 2-associated transforming growth factor-beta. We conclude that impaired cellular stress-responsiveness, aggravated by the inflammatory milieu at the onset of type 1 diabetes, contributes to disease manifestation.