Objective: The objective of the study was to quantify insulin resistance in type 1 diabetes patients by estimated glucose disposal rate (eGDR), according to the presence or absence of the metabolic syndrome, and its relationship with chronic complications. Design: This was a cross-sectional study in 91 patients with type 1 immune-mediated diabetes managed at an outpatient endocrinology clinic. All participants were Caucasians aged 18 yr or older with type 1 diabetes duration of more than 6 months who had completed the study protocol. Results: Twenty-nine patients met metabolic syndrome criteria, yielding a prevalence of 31.9%. Although no differences in insulin requirements were found between diabetic patients with and without metabolic syndrome, lower eGDR levels, indicating greater insulin resistance, were observed in metabolic syndrome patients compared with those without (6.19 ± 1.5 mg/kg -1 • min-1 vs. 9.93 ± 1.6 mg/kg-1 • min-1) (P < 0.001). An eGDR level less than 8.77 mg/kg -1 • min-1 showed 100%sensitivity and 85.2% specificity for metabolic syndrome diagnosis. All patients with diabetes complications had eGDR values below 8.16 mg/kg-1 • min -1. eGDR level was significantly lower in patients with diabetic retinopathy (5.97 ± 1.2 mg/kg-1 • min-1), diabetic neuropathy (5.06 ± 0.4 mg/kg-1 • min -1), or diabetic nephropathy (5.79 ± 1.5 mg/kg-1 • min-1) compared with those without (9.38 ± 2.0 mg/kg-1 • min-1, P < 0.001; 9.26 ± 2.0 mg/kg-1 • min-1, P < 0.001; and 9.19 ± 2.2 mg/kg-1 • min-1, P < 0.001). Conclusions: Insulin resistance is common in type 1 diabetes patients and is associated with microvascular complications. eGDR, as an insulin resistance marker, provides more useful information than other classical variables such as insulin requirements. Copyright © 2009 by The Endocrine Society.