The aim of this retrospective study was to investigate the accuracy of using preoperative data for the prediction of conversion from laparoscopic to open cholecystectomy in patients operated on for acute calculous cholecystitis. Laparoscopic cholecystectomy was scheduled in eighty-nine of 184 consecutive patients with acute calculous cholecystitis who underwent urgent or early cholecystectomy without bile duct exploration in our department between 1991 and 1998. The correlation between 11 preoperative clinical, laboratory and ultrasonographic variables, and the rate of conversion to open cholecystectomy was studied. Among the 11 variables tested, age and leukocyte count were independent factors of predictive significance. These two factors were used for constructing an additive prognostic index for conversion to open cholecystectomy. Thus, three groups of patients could be identified having a 10%, 30-70% or over 88% risk of conversion. Logistic regression analysis permits accurate preoperative identification of unsuccessful laparoscopic cholecystectomy in patients with acute calculous cholecystitis.