The incidence of acute acalculous cholecystitis (AAC) is increasing and associated mortality is high. Biliary stasis and sludge formation are probably important factors in the pathogenesis of this disease. No data concerning the dynamics of these changes in the early phase of intensive care therapy are available. The gallbladders of 20 patients treated after major abdominal surgery in the surgical intensive care unit (SICU) with mechanical ventilation and without enteral feedings were therefore observed sonographically during the first 5 postoperative days in a prospective observational study. 20 patients treated on a regular ward after major abdominal surgery also not receiving any enteral nutrition served as control group. 24 hours after admission to the intensive care unit and on all subsequent days of observation the gallbladders of the patients in the SICU-group were significantly larger than in the control group. Sludge also appeared earlier and more frequently in the gallbladders of the SICU-patients. Lack of enteral feedings alone cannot explain these results. Positive-pressure ventilation and medications used in SICU are most likely responsible for the observed differences. Besides the necessity to make the diagnosis of AAC as early as possible, it appears to be worthwhile to investigate measures of prophylaxis. Since gallbladder distension in patients treated in SICU can be already observed on the first postoperative day it seems to be reasonable to initiate a regimen of prophylactic measures (e.g. with cholecystokinin or ceruletide) early in the course of ICU-therapy.