UNLABELLED: Assessment of beta-cell function in type 1 diabetes (T1D) has important implications in both clinical and research settings. Studies demonstrating the extent to which puberty influences C-peptide levels are scarce. The aim of this study was to evaluate the influence of pubertal stage, along with age and body mass index (BMI), on multiple C-peptide measures at T1D diagnosis. This study included 275 consecutive children aged between 1 and 18 years with newly diagnosed T1D. Fasting, prandial, and area under the curve (AUC) C-peptide, estimated using fasting and prandial C-peptide levels, were analyzed. Generalized linear regression models were utilized. Median age at diagnosis was 7.9 (1.1-17.3) years, and mean BMI standard deviation score (SDS) was - 0.4 ± 1.4. Of the patients, 66% were prepubertal. Median fasting and prandial C-peptide levels at diagnosis were 0.26 (0.05-1.8) ng/mL and 0.43 (0.05-3) ng/mL, respectively. Fasting C-peptide was almost perfectly correlated with prandial C-peptide (r CONCLUSIONS: Pubertal stage significantly and independently impacts C-peptide levels at T1D diagnosis along with body mass index and age. The adjustments demonstrating the extent to which puberty influences C-peptide levels in new-onset T1D are presented. Our observations underline the existence of distinct endotypes of T1D characterized by differing immunopathological courses. WHAT IS KNOWN: • Throughout the evaluation of beta-cell function at T1D diagnosis, it is essential to consider the factors influencing C-peptide levels. WHAT IS NEW: • While age and BMI at diagnosis are associated with beta-cell function, our findings set the stage for a greater understanding of the disease process with additional findings regarding puberty, supporting the existence of endotypes of T1D.