Objective: To stop smoking is commonly associated with significant weight gain, but the mechanisms for this are poorly understood. We assessed the effects of smoking cessation on body weight, insulin sensitivity, b-cell function, and appetite. Subjects and methods: Twenty-seven long-term smokers (nZ27; nine females/18 males, 28G1 years, 22.9G0.6 kg/m 2 )attending an ambulatory smoking cessation program in a community hospital in Vienna, Austria were examined at baseline (Visit A; still smoking) and after a minimum of 3 months of smoking abstinence (Visit B; nZ14); relapsed smokers were not followed up. Participants underwent 3-h oral glucose tolerance tests and body composition measurements at each study visit. Fasting (QUICKI) and dynamic (oral glucose insulin sensitivity (OGIS)) insulin sensitivity and b-cell secretion (insulinogenic index 140 (IGI40)) were calculated. Food intake was quantified with a free choice buffet. Fasting plasma concentrations of neuropeptide-Y (NPY), peptide-YY (PYY), glucagon-like peptide 1 (GLP1), leptin, ghrelin, and visfatin were measured. Results: After O3 months' smoking abstinence, body weight, and fat mass were increased (C4 and C22% respectively, P!0.05) and fasting insulin sensitivity deteriorated (QUICKI: post, 0.37G0.02 vs baseline, 0.41G0.2; P!0.05), while OGIS remained unchanged throughout. IGI40 increased by 31% after O3 months' smoking abstinence (P!0.01). Carbohydrate ingestion increased after stopping smoking (P!0.05). NPY fasting levels were increased after O3 months (P!0.05), PYY, GLP1, leptin, ghrelin, and visfatin were unchanged. Conclusion: Smoking cessation is associated with transient metabolic changes including increased b-cell secretion in response to glucose and fasting insulin resistance. These alterations may be associated with or contribute to the body weight gain after smoking cessation.