Vitamin D and ω-3 fatty acids (ω-3) co-supplementation potentially improve type 1 diabetes (T1D) by attenuating autoimmunity and counteracting inflammation.This cohort study preliminary to a randomized control trial (RCT) is aimed to evaluate, in a series of T1D children assuming Mediterranean diet and taking cholecalciferol 1000U/day fromT1D onset, if ω-3 co-supplementation preserves the residual endogen insulin secretion (REIS).Therefore, 22 new onsets of 2017 received ω-3 [eicosapentenoic acid (EPA) plus docosahexaenoic acid (DHA), 60mg/kg/day], and were compared retrospectively vs. 37 previous onsets without ω-3 supplementation. HbA1c%, daily insulin demand (IU/Kg/day) and IDAA1c, a composite index (calculated as IU/Kg/dayx4 + HbA1c%) as surrogate of REIS, were evaluated at recruitment (T0) and 12 months later (T12). In the ω-3 supplemented group, dietary intakes were evaluated at T0 and T12. As outcome, decreased insulin demand (p<0.01), particularly pre-meal boluses (p<0.01), and IDAA1c (p<0.05), were found in the ω-3 group while HbA1c% were not different in the two groups. Diet analysis, at T12 vs. T0, showed that the intake of arachidonic acid (AA) was decreased (p<0.01) in the ω-3 supplemented group while other nutrients were unchanged; at T0, the AA intake was inversely correlated with HbA1c% (p<0.05; r;.0.411). In conclusion, the results suggest that vitamin D plus ω-3 co-supplementation and AA reduction in Mediterranean diet display benefits for T1D children and deserve further investigation.