Introduction and objective. Continuous subcutaneous insulin infusion (CSII) via personal insulin pump is a valuable therapeutic tool in T1DM patients. However, adherence to recommended CSII- related behaviours may be of concern to young adults with intensive, variable daily activities (students, young professionals). The aim of this observational study was to estimate treatment outcomes in young adult patients with T1DM, and compare them with older individuals. Materials and methods. Overall, 140 adults with T1DM on CSII were examined, divided into 2 subgroups: 77 patients younger than 26 years of age (mean 20.6 years) and 63 older subjects (mean 39.0). We compared the glycaemic control in both groups of T1DM subjects and analyzed treatment attitudes to identify potentially modifiable behaviours influencing the efficacy of the treatment. Results. The younger individuals were characterized by significantly worse treatment outcomes, compared to the older ones: the mean HbA1c levels were 7.6 ± 1.3% and 6.9±1.3% (p=0.00001), while the mean glucose levels based on glucometer downloads were 161±33.6 mg/dL and 136±21.8 mg/dL (p=0.00001), respectively. The frequency of self-monitoring of blood glucose (SMBG) was lower in younger individuals (5.3±2.1 vs. 7.0±2.8 daily, p=0.0005, respectively); they were also less frequently used advanced pump functions, e.g. the bolus calculator (48% vs. 67% users, p=0.0014, respectively). Conclusions. The efficacy of CSII treatment observed in young T1DM adults was worse than in older patients. The reason for this phenomenon remains unclear, it may be due simply to age-dependend behaviours, to social environment, or both.