Objective. Recent studies have shown that high interleukin-6 (IL-6) secretion may aggravate insulin resistance in pregnancy and participate in the pathogenesis of gestational diabetes mellitus (GDM). The aim of this study was to determine whether the presence of GDM is associated with elevated IL-6 concentrations and whether this association remains after delivery, independent of body mass index. Design. Longitudinal study. Setting. Hospital-based. Sample. Forty-seven women were screened for GDM with a 75g oral glucose tolerance test at 26.1±3.7 weeks of pregnancy following the Canadian Diabetes Association guidelines (20 GDM, 27 control subjects). Main outcome measures. Interleukin-6 levels were measured by ELISA at the time of GDM screening and two months post-partum. Results. Interleukin-6 concentrations were significantly higher in women with GDM compared with control women at the time of GDM screening (1.47±0.72 vs. 0.90±0.32pg/mL, p≤0.01). Similar results were obtained two months post-partum, where IL-6 levels remained significantly higher in women with GDM compared with control women (1.88±0.85 vs. 1.41±0.87pg/mL, p≤0.05). Interleukin-6 concentrations were significantly correlated with the Matsuda insulin sensitivity index, measured at the two time points (r=–0.60, p≤0.01 and r=–0.34, p≤0.05). The Matsuda insulin sensitivity index was an independent and significant predictor of IL-6 concentrations at the time of GDM screening, explaining 35.6% of the variance (p≤0.0001) in this variable. IL-6 concentration measured at GDM screening was identified as an independent and significant predictor of post-partum IL-6 concentrations, explaining 28.6% of the variance (p≤0.001). Conclusions. These results show that GDM is associated with elevated IL-6 levels independent of obesity levels, both during pregnancy and after delivery.