Background: Obesity is a major modifiable risk factor for type 2 diabetes, cardiovascular diseases and increased mortality. Bariatric surgery is followed by long-term weight loss and decreased overall mortality.Aim: To investigate the impact of weight loss following gastric bypass surgery on lipoproteins, LDL and HDL quality, inflammatory markers, adipose cell size and adipose tissue inflammation.Methods: 27 obese patients, 11 with type 2 diabetes, were recruited. Lipoproteins, glucose, insulin and inflammatory biomarkers were measured before, 6 and 12 months after surgery. HDL and LDL in vitro susceptibility to oxidation was assessed. We also studied correlations between different parameters. The effect of weight loss on adipose cell size and adipose tissue inflammation was assessed in a subgroup of patients 6 months after surgery.Results: After surgery BMI, fasting blood glucose, HbA1C, insulin, triglycerides, HDL-C, Small-dense LDL apoB, glycated apoB, TNF-α, IL-6, hs-CRP, resistin, MCP-1, ICAM, VCAM, E-selectin, adiponectin, leptin and ASP decreased significantly. Susceptibility of LDL to oxidation in vitro decreased 12 months after bariatric surgery and that of HDL also decreased but the reduction was only statistically significant in non-diabetic patients. There was a significant reduction in adipocyte size and the change in average adipocyte area correlated with the change in BMI after surgery. The CD68 [KP1] staining macrophage numbers and percentage of adipose tissue area that stains for TNF-α significantly decreased following surgery.Conclusions: Lipoproteins, glucose metabolism and vascular inflammatory biomarkers improved with weight loss following surgery. These changes are likely to contribute to morbidity and mortality improvement after bariatric surgery. Our results suggest that non-diabetic patients may draw a greater benefit from bariatric surgery than those with type 2 diabetes suggesting a role for early gastric bypass surgery before complications of obesity develop. Adipose cell size and adipose tissue inflammation decreased significantly 6 months after surgery.