Obesity is associated with numerous inflammatory conditions including atherosclerosis, autoimmune disease and cancer. Although the precise mechanisms are unknown, obesity-associated rises in TNF-alpha, IL-6 and TGF-beta are believed to contribute. Here we demonstrate that obesity selectively promotes an expansion of the Th17 T-cell sublineage, a subset with prominent pro-inflammatory roles. T-cells from diet-induced obese mice expand Th17 cell pools and produce progressively more IL-17 than lean littermates in an IL-6-dependent process. The increased Th17 bias was associated with more pronounced autoimmune disease as confirmed in two disease models, EAE and trinitrobenzene sulfonic acid colitis. In both, diet-induced obese mice developed more severe early disease and histopathology with increased IL-17(+) T-cell pools in target tissues. The well-described association of obesity with inflammatory and autoimmune disease is mechanistically linked to a Th17 bias.