-Background -Obesity is one of the world's greatest health problems. The Roux-en-Y gastric bypass is the gold standard treatment for severe obesity. Surgery in obese patients has an acceptable level of morbidity and mortality. The superobese patient, a subcategory of severe obese patients with a high surgical risk has not yet been analyzed as a group. Methods -A retrospective and prospective cohort study was conducted enrolling 135 patients submitted to Roux-en-Y gastric bypass for treatment of severe obesity at the "Hospital das Clínicas", Federal University of Pernambuco, Recife, PE, Brazil, between November 1997 and September 2003. The independent variables were possible risk factors of adverse outcomes: age, gender, weight, body mass index, diabetes, hypertension, hypercholesterolemia, sleep apnea, cardiopathy/coronariopathy, pneumopathy or any other co-morbidity. The dependent variables were major complications, minor complications and death. Results -Diabetes (RR = 1.6 and CI = 1.02-2.40) and sleep apnea (RR = 1.8 and CI = 1.18-2.64) were associated to minor complications. Cardiopathy/coronariopathy were associated with major complications (RR = 5.42 and CI = 1.22-2.40) and death (RR = 16.25 and CI = 3.00-87.95). BMI ≥55 kg/m 2 was associated with minor complications (RR = 1.58 and CI = 1.04-2.40), major complications (RR = 3.17 and CI = 1.03-9.80) and death (P = 0.007). After logistic regression, the body mass index ≥55 kg/m 2 remained as a strong risk factor of death (OR = 3.6 and CI = 1.05-12.32). Conclusions -The body mass index ≥55 kg/m 2 was the main risk factor for severe complications and death. Other risk factors affecting the outcome were diabetes, obstructive sleep apnea and cardiopathy/coronariopathy.