Introduction: Numerous studies have demonstrated the clinical benefits of using continuous glucose monitoring (CGM) systems among patients with type 1 diabetes (T1D). Aim of the study was to assess the effectiveness of CGM on metabolic control in children with T1D and well-controlled disease prior to the study. Material and methods: This prospective analysis included 99 children (46 girls) at the median age of 11.23 years and diabetes duration of at least 1 year (median: 5.16 years), generally well controlled metabolically (median HbA 1c : 7.0%), and treated with continuous subcutaneous insulin infusion (CSII). The patients had used CGM for at least 150 days. We analysed the participants in subgroups based on baseline HbA 1c < 7%, ≥ 7%, age, and sex. Results: Children with baseline HbA 1c < 7% were characterized by significantly increased HbA 1c after the median of 273 days (217; 320) of CGM usage (6.3% vs. 6.6%, respectively; p = 0.002). No significant change in HbA 1c was noted in children with baseline HbA 1c ≥ 7% (7.5% vs. 7.4%, respectively; p = 0.191), but 20% of the group reached the target of HbA 1c < 7.0%. The analysis of CGM data revealed that no group achieved the CGM targets of good metabolic control. Total daily insulin requirements remained stable in both groups (p = 0.752; p = 0.274), but the amount of basal insulin increased statistically in both groups (p = 0.009; p ≤ 0.001). Conclusions: The application of CGM provides detailed information concerning glycaemic control and is beneficial in some, but not all, T1D children with good diabetes control.