Objective: We aimed to determine myo-inositol oxygenase (MIOX) activity in gestational diabetes mellitus (GDM). Material and Methods: The insulin, HbA1c, and MIOX levels of 80 pregnant women were analysed after 75 g OGTT. Group I included patients with no risk factor for GDM, Group II: patients with high risk for GDM, and Group III: GDM patients. Results: Fasting plasma glucose and Homeostatic Model Assessment for Insulin Resistance index were significantly higher in GDM cases (Group 3). Regarding the MIOX levels, significantly higher levels were recorded at 0-h in Group 3 compared to Groups 1 and 2. Significant alteration in MIOX activity was found between 0- and 2-h in Group 3 compared to Groups 1 and 2 (p < .029). Conclusions: MIOX levels were higher in GDM cases so, it may be have a role in myo-inositol catabolism.