High-mobility group box-1 (HMGB1) is a pro-inflammatory cytokine that may contribute to the pathogenesis of micro- and macrovascular complications commonly observed in diabetes. We investigated whether HMGB1 is associated with: i) markers of low-grade inflammation (LGI) and endothelial dysfunction (ED) and pulse pressure (PP, a marker of arterial stiffness); ii) prevalent nephropathy, retinopathy and cardiovascular disease (CVD) in type 1 diabetes; and iii) the potential mediating roles of LGI, ED and PP therein. This was a cross-sectional nested case-control study of 463 patients (226 women; mean age 40±10 years) with type 1 diabetes from the EURODIAB Prospective Complications Study. We used linear and binary or multinomial logistic regression analyses adjusted for traditional risk factors. Serum Ln-HMGB1 levels were positively associated with LGI and ED (standardised β=0.07 (95% confidence interval (CI): 0.02-0.12) and β=0.08 (95% CI: 0.02-0.14) respectively), but not with PP. Higher Ln-HMGB1 (per unit) was associated with greater odds of micro- and macroalbuminuria: odds ratio (OR)=1.24 (95% CI: 0.90-1.71) and OR=1.61 (95% CI: 1.15-2.25) respectively, P for trend=0.004. Further adjustments for LGI or ED did not attenuate these associations. No such associations were found between Ln-HMGB1 and estimated glomerular filtration rate (eGFR), retinopathy or CVD, however. In type 1 diabetes, higher serum HMGB1 levels are associated with greater prevalence and severity of albuminuria, though not with eGFR, retinopathy and CVD. Prospective studies are needed to clarify the causal role of HMGB1, if any, in the pathogenesis of vascular complications in type 1 diabetes.