The level of C-peptide can identify individuals most likely to respond to immune interventions carried out to prevent pancreatic β-cell damage. The aim of the study was to evaluate factors associated with C-peptide levels at type 1 diabetes (T1D) diagnosis.This study included 1098 children aged 2-17 with newly recognized T1D. Data were collected from seven Polish hospitals. The following variables were analyzed: date of birth, fasting C-peptide, HbA1c, sex, weight, height, pH at diabetes onset.A correlation was observed between fasting C-peptide level and BMI-SDS (p = 0.0001), age (p = 0.0001), and HbA1c (p = 0.0001). The logistic regression model revealed that fasting C-peptide ≥0.7 ng/ml at diabetes diagnosis was dependent on weight, HbA1c, pH and sex (p < 0.0001). Overweight and obese children (n = 124) had higher fasting C-peptide (p = 0.0001) and lower HbA1c (p = 0.0008) levels than other subjects. Girls had higher fasting C-peptide (p = 0.036) and higher HbA1c (p = 0.026) levels than boys.Obese and overweight children are diagnosed with diabetes at an early stage with largely preserved C-peptide levels. Increased awareness of T1D symptoms as well as improved screening and diagnostic tools are important to preserve C-peptide levels. There are noticeable gender differences in the course of diabetes already at T1D diagnosis.