This study was conducted to assess the prognostic value of obesity in acute pancreatitis and to determine the role played by obesity-associated diseases in the course of the disease. We prospectively studied 49 patients with acute pancreatitis who were divided into three groups according to their body mass index (BMI). There were 22 patients in group I (BMI < or = 25 kg/m2, normal or low weight); 15 in group II (BMI >25 and < or = 29 kg/m2, overweight); and 12 in group III (BMI >29 kg/m2, obese). Other anthropometric parameters also were measured. The severity of pancreatitis was assessed according to the Atlanta classification system. Systemic complications were significantly more common among obese than nonobese patients (p < 0.05). Patients with severe pancreatitis had a higher body-fat percentage, measured by the subscapular skin-fold thickness, and a larger abdominal circumference than patients with mild pancreatitis. Although hypertensive or diabetic patients developed more systemic complications, the multivariate analysis demonstrated that the presence of these underlying diseases did not modify the prognostic role of obesity in acute pancreatitis. We conclude that obesity is a prognostic factor of outcome in acute pancreatitis. Obesity-associated diseases do not vary the prognostic value of obesity. It seems that truncal adiposity is the kind of obesity related to worse outcome of acute pancreatitis.