Adipose tissue dysfunction plays a key role in the development of the metabolic abnormalities characteristic of type 2 diabetes (T2DM) and participates actively in lipid metabolism. Adiponectin, found abundantly in circulation and a marker of adipose health, is decreased in obese persons with T2DM. We investigated whether the changes in adiponectin with an intensive lifestyle intervention (ILI) for weight loss could potentially mediate the increase in low HDL-cholesterol (HDL-C) with ILI. Adiponectin and its fractions were determined using an ELISA with selective protease treatment in 1,397 participants from Look AHEAD, a trial examining whether ILI will reduce cardiovascular events in overweight/obese subjects with T2DM when compared with a control arm, diabetes support and education (DSE). Multivariable regression and mediational analyses were performed for adiponectin and its high-molecular-weight (HMW) and non-HMW fractions. ILI increased baseline HDL-C by 9.7% and adiponectin by 11.9%; changes with DSE were 1.3% and 0.2%, respectively (P < 0.0001). In a model including changes in weight, fitness, triglycerides, and glucose control and that adjusted for demographics and medical history, adiponectin changes remained significantly associated with HDL-C change. Data supported the contribution of changes in both HMW- and non-HMW-adiponectin to the improvement in HDL-C with ILI.