Aim. - Obesity is associated with the development of metabolic complications such as insulin resistance (IR). The mechanisms leading to IR remain unclear. This study aimed to investigate the relationship between adipose tissue fibrosis and IR in obese patients before and after bariatric surgery. Methods. - Thirty-five obese patients awaiting bariatric surgery (12 with type 2 diabetes) were included in the study. Non -diabetic patients were classified as either insulin -sensitive (n = 11) or insulin -resistant (n = 12), based on the Matsuda insulin sensitivity index (ISImatsuda). Homoeostasis model assessment (HOMA-IR) was used for longitudinal evaluation of insulin resistance. Fibrosis was quantified by Masson's trichrome staining on microscopy, and mRNA levels of fibrosis-related genes were examined in subcutaneous (SAT) and visceral adipose tissue (VAT) biopsies collected during and 6 months after bariatric surgery (SAT only). Results. - Despite their similar age, body mass index and fat mass, SAT fibrosis was significantly higher in diabetic vs insulin -sensitive patients (P < 0.05), and associated with IR as assessed by both ISImatsuda (r= 0.417, P = 0.038) and HOMA-IR (r= 0.464, P = 0.007) at baseline, whereas VAT fibrosis was not. Six months after surgery and significant weight loss, fibrosis levels remained unchanged in SAT, although IR was significantly reduced in all groups (P < 0.0001). No correlation was found between SAT fibrosis and IR after surgery. Conclusion. - Overall, these results show a significant but, most likely, transient association between SAT fibrosis and IR in obese humans. (C) 2016 Elsevier Masson SAS. All rights reserved.