Since insulin resistance (IR) is highly prevalent in patients with systemic lupus erythematosus (SLE) and rheumatoid arthritis (RA), we aimed to determine whether differences in IR between the two conditions exist.Cross-sectional study that encompassed 413 non-diabetic subjects, 186 SLE and 227 RA. Glucose, insulin and C-peptide serum levels, as well as IR by the homeostatic model assessment (HOMA2) were studied. A multivariable regression analysis was performed to evaluate the differences in IR indexes between patients with SLE and RA, and also to determine if IR risk factors or disease-related characteristics are differentially associated with IR in both populations.The insulin:C-peptide molar ratio was upregulated in RA compared to SLE patients (beta coef. 0.009 [95%CI 0.005-0.014], p=0.000) after multivariable analysis. HOMA2 indexes related to insulin sensitivity were found to be lower (HOMA2-S% beta coef. -27 [95%CI -46- -9], p=0.004) and beta cell function showed higher IR indexes (HOMA2-B% beta coef. 38 [95%CI 23-52], p=0.000) in RA than in SLE patients after multivariable analysis. RA patients more often fulfilled the definition of IR than those with SLE (odds ratio 2.15 [95%CI 1.25-3.69], p=0.005). The size effect of IR factors on IR indexes was found to be equal in both diseases.IR sensitivity is lower and beta cell function is higher in RA than in SLE patients. The fact that traditional IR factors have an equal effect on IR in both SLE and RA supports the contention that these differences are related to the diseases themselves.