A role for high leptin levels in the proinflammatory state associated with obesity has been proposed on the basis of observational studies, but a recent interventional study employing administration of long-acting pegylated leptin resulting in very high pharmacologic levels in obese subjects did not support this idea. These interventional studies have not yet been independently confirmed, however, and varying levels and duration of hyperleptinemia as well as the presence of comorbidities such as diabetes have not yet been investigated as potential effect modifiers. We performed three interventional studies involving administration of recombinant methionyl human leptin (r-metHuLeptin) to lean, otherwise healthy obese, and obese diabetic subjects to investigate whether increasing circulating leptin levels over a wide spectrum of values (from low physiologic to high pharmacologic) would alter serum levels of inflammatory markers and other cytokines important in the T helper cell response. Increasing leptin levels from low physiologic to high physiologic in lean men and from higher physiologic to low pharmacologic in obese men over 3 d did not alter serum interferon-gamma, IL-10, TNF-alpha, monocyte chemoattractant protein-1, or soluble intercellular adhesion molecule-1. In obese subjects with type 2 diabetes mellitus, the administration of r-metHuLeptin for 4 or 16 wk, resulting in high pharmacologic leptin levels, did not activate the TNF-alpha system or increase cytokines or inflammatory markers, including IL-10, IL-6, C-reactive protein, monocyte chemoattractant protein-1, and soluble intercellular adhesion molecule-1. These findings do not support an etiopathogenic role for leptin in proinflammatory states associated with leptin excess such as obesity and have direct relevance for the potential future therapeutic use of r-metHuLeptin in humans.