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Purpose: 2-deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) uptake is a marker of metabolic activity and is therefore used to measure the inflammatory state of several tissues. This radionuclide marker is transported through the cell membrane via glucose transport proteins (GLUTs). The aim of this study is to investigate whether insulin resistance (IR) or inflammation plays a role in [18F]FDG uptake in adipose tissue (AT). Procedures: This study consisted of an in vivo clinical part and an ex vivo mechanistic part. In the clinical part, [18F]FDG uptake in abdominal visceral AT (VAT) and subcutaneous AT (SAT) was determined using PET/CT imaging in 44 patients with early type 2 diabetes mellitus (T2DM) (age 63 [54–66] years, HbA1c [6.3 ± 0.4 %], HOMA-IR 5.1[3.1–8.5]). Plasma levels were measured with ELISA. In the mechanistic part, AT biopsies obtained from 8 patients were ex vivo incubated with [18F]FDG followed by autoradiography. Next, a qRT-PCR analysis was performed to determine GLUT and cytokine mRNA expression levels. Immunohistochemistry was performed to determine CD68+ macrophage infiltration and GLUT4 protein expression in AT. Results: In vivo VAT [18F]FDG uptake in patients with T2DM was inversely correlated with HOMA-IR (r = − 0.32, p = 0.034), and positively related to adiponectin plasma levels (r = 0.43, p = 0.003). Ex vivo [18F]FDG uptake in VAT was not related to CD68+ macrophage infiltration, and IL-1ß and IL-6 mRNA expression levels. Ex vivo VAT [18F]FDG uptake was positively related to GLUT4 (r = 0.83, p = 0.042), inversely to GLUT3 (r = − 0.83, p = 0.042) and not related to GLUT1 mRNA expression levels. Conclusions: In vivo [18F]FDG uptake in VAT from patients with T2DM is positively correlated with adiponectin levels and inversely with IR. Ex vivo [18F]FDG uptake in AT is associated with GLUT4 expression but not with pro-inflammatory markers. The effect of IR should be taken into account when interpreting data of [18F]FDG uptake as a marker for AT inflammation. [ABSTRACT FROM AUTHOR]