Aims/hypothesis. The humoral part of the immune system, including autoantibodies, is known to predict manifest Type I (insulin-dependent) diabetes mellitus in first-degree relatives but the cell-mediated immune process preceding the manifest disease still is not known. The aim of this investigation was to estimate the immunological balance of Th-like lymphocytes (Th1/Th2) in high-risk first-degree relatives of Type I diabetic children.¶Methods. Peripheral blood mononuclear cells (PBMC) from 21 healthy high-risk first-degree relatives (ICA ≥ 20) were examined and compared with the response seen in PBMC from children with newly diagnosed Type I diabetes and healthy control subjects of similar age, sex and HLA-type. Expression of interleukin-4 (IL-4) and interferon-γ (IFN-γ) mRNA were determined by RT-PCR and as protein by ELISPOT after stimulation with specific epitopes of GAD₆₅ (a. a. 247–279, 509–528, 524–543), bovine serum albumin, the ABBOS peptide (a. a. 152–169) and insulin.¶Results. High-risk relatives had a high ratio of IFN-γ:IL-4 compared with both diabetic children (p = 0.0005) and healthy control subjects (p = 0.004). Production of IFN-γ seen in high-risk relatives was negatively correlated to production of GADA (r = –0.44, p = 0.05). The high concentration of IFN-γ from high-risk relatives, decreased after stimulation with peptides of GAD₆₅, the ABBOS peptide, BSA and insulin. Increased secretion of IL-4 was observed after stimulation with two peptides of GAD₆₅ (a. a. 509–528 and 524–543), the ABBOS peptide and insulin.¶Conclusion/interpretation. Overwhelming production of IFN-γ seen in peripheral blood mononuclear cells from high-risk first-degree relatives of children with Type I diabetes suggests a Th1-like immune deviation in the prediabetic phase. [Diabetologia (2000) 43: 742–749]