Ninety-nine patients with acute pancreatitis in whom body mass index (BMI = weight (kg)/height2 (m2)) was measured were studied prospectively to determine the importance of obesity as a prognostic factor in this disease. Of 19 obese patients (BMI > or = 30 kg/m2), 12 developed severe pancreatitis; seven had abscesses, of whom five died, and two further patients died. In 80 non-obese patients, the incidence of severe pancreatitis (n = 5), abscess formation (n = 4) and death (n = 4) was significantly less (P = 0.0007). The mean(s.d.) BMI of 17 patients with severe acute pancreatitis was significantly higher than that in 82 patients with mild acute disease (31.2(5.6) versus 23.3(5.6) kg/m2, P < 0.001). As a single prognostic factor, obesity had a sensitivity of 63 per cent and a specificity of 95 per cent for predicting disease severity. When five obese women with gallstone pancreatitis were excluded, the sensitivity of obesity increased to 86 per cent. Severe pancreatitis occurred in all eight obese patients with disease of an alcoholic aetiology. These data suggest that increased fat deposits in the peripancreatic and retroperitoneal spaces in obese patients may increase the risk of peripancreatic fat necrosis, abscess and death. Consideration should be given to including obesity as a prognostic factor in acute pancreatitis.