['To investigate the relationship between physiological serum total bilirubin concentrations and serum C-peptide levels in Korean patients with Type 2 diabetes.', 'A total of 588 patients with Type 2 diabetes were investigated in this cross-sectional study. Fasting C-peptide level, 2-h postprandial C-peptide level and ΔC-peptide (postprandial C-peptide minus fasting C-peptide) level were measured in all patients.', 'Fasting C-peptide level, postprandial C-peptide level and ΔC-peptide level tended to be higher in patients with higher bilirubin concentrations. Partial correlation analysis showed that serum bilirubin levels were significantly correlated with fasting C-peptide level (r\xa0=\xa00.159, P < 0.001), postprandial C-peptide level (r\xa0=\xa00.209, P < 0.001) and ΔC-peptide level (r\xa0=\xa00.186, P < 0.001) after adjustment for other covariates. In the multivariate model, the association between serum bilirubin concentrations and serum C-peptide levels remained significant after adjustment for confounding factors including age, gender, familial diabetes, hypertension, hyperlipidaemia, BMI, HbA1c , duration of diabetes and associated liver function tests (fasting C-peptide level: β\xa0=\xa00.083, P\xa0=\xa00.041; postprandial C-peptide level: β\xa0=\xa00.106, P\xa0=\xa00.005; ΔC-peptide level: β\xa0=\xa00.096, P\xa0=\xa00.015, respectively).'] Are serum bilirubin concentrations positively associated with serum C-peptide levels in patients with Type 2 diabetes?