Context: The effects of medical and surgical treatments for obesity on peptide YY (PYY) levels, in patients with similar weight loss, remain unclear. Objective: The objective of the study was to assess PYY and appetite before and after Roux-en-Y gastric bypass (RYGB), sleeve gastrectomy (SG), and medical treatment (MED). Design: This was a prospective, controlled, nonrandomized study. Setting: The study was conducted at the Departments of Nutrition and Digestive Surgery at a university hospital. Participants: Participants included three groups of eight patients with similar body mass indexes (RYGB 37.8 ± 0.8, SG 35.3 ± 0.7, and MED 39.1 ± 1.7 kg/m2, P = NS) and eight lean controls (body mass index 21.7 ± 0.7 kg/m 2). Main Outcome Measures: Total plasma PYY, hunger, and satiety visual analog scales in fasting and after ingestion of a standard test meal were measured. Results: At baseline there were no differences in the area under the curve (AUC) of PYY, hunger, or satiety in obese groups. Two months after the interventions, RYGB, SG, and MED groups achieved similar weight loss (17.7 ± 3, 14.9 ± 2.4, 16.6 ± 4%, respectively, P = NS). PYY AUC increased in RYGB (P < 0.001) and SG (P < 0.05) and did not change in MED. PYY levels decreased at fasting, 30 min, and 180 min after a standard test meal in MED(P<0.05). Hunger AUC decreased in RYGB (P < 0.05). Satiety AUC increased in RYGB (P < 0.05) and SG (P < 0.05). Appetite did not change in MED. PYY AUC correlated with satiety AUC (r = 0.35, P < 0.05). Conclusion: RYGB and SG increased PYY and reduced appetite. MED failed to produce changes. Different effects occur despite similar weight loss. This suggests that the weight-loss effects of these procedures are enhanced by an increase in PYY and satiety.