Background: Type 1 diabetes mellitus (T1DM) incidence is currently increasing worldwide, and different environmental players along with genetic predisposition, could be involved as powerful triggers of this disease onset. In this study, we aim to shed the light on the relationship between 25OHD deficiency and T1DM. Patients and methods: A case-control study was laid out to compare serum 25OHD level between T1DM patients and controls. A total of 147 T1DM patients aged under 19 years old were recruited from our Endocrinology-Diabetology and Nutrition department between October 2014 and December 2019. A total of 147 controls were randomly enlisted from clinical biochemistry laboratory of our center, and were carefully matched. The levels of 25OHD in the serum were determined in T1DM patients and nondiabetic controls. Results: Average serum 25OHD concentration was established in both groups; reaching 19,29 ±6,13 ng/ml in the control arm and 15,02 ± 6,48 ng/ml in the selected group with T1DM independently of the disease duration. However, the mean serum 25OHD concentration was not significantly different between the two T1DM subgroups according to diabetes duration below or above 5 years, and 25OHD concentration remained lower either in winter or summer months. A negative correlation was noticed between HbA1c and serum 25 OHD concentration in T1DM patients and was statistically significant (p<0,05). Conclusion: Key messages on the importance of vitamin D status, particularly in diabetic children and adolescents, should be spread widely in order to start a suitable vitamin supplementation, and establish guidelines regarding its timing at adequate recommended doses..