Objective <p>Many patients with long-standing type 1 diabetes (T1D) have remaining low levels of C-peptide, i.e. and indirect sign of remaining functional beta-cells. This study focused on identifying differences in immunological and inflammatory biomarkers in patients with longstanding T1D and remaining C-peptide.</p> Research design and methods <p>Adult patients (<i>n</i> = 120) with long-standing T1D (≥ 10 years) and healthy controls (HC) (<i>n</i> = 50) were recruited at Uppsala University Hospital. Residual beta-cell function was determined with an ultrasensitive C-peptide ELISA under fasting conditions. T1D patients were divided into two groups (C-peptide positive vs. C-peptide negative). Using the OLINK Explore Inflammation proximity extension assay (PEA), 368 circulating immunological and inflammatory biomarkers were analyzed in plasma.</p> Results <p>The three groups could not be distinguished by principal component analysis and when correcting for multiple testing we found no differences in circulating biomarkers. However, based on uncorrected p-values there were six biomarkers that were different when comparing all T1D patients with HC and eight markers that were different when comparing C-peptide positive vs. negative T1D patients.</p> Conclusion <p>A wide inflammatory assay analysis cannot distinguish patients with longstanding T1D and remaining C-peptide from patients with a complete loss of C-peptide nor from HC.</p>