Thirty-two intensive care unit patients (78% on long-term total parenteral nutrition) suspected of having acute acalculous cholecystitis (AAC) were studied prospectively. All of these patients underwent abdominal ultrasonography and cholescintigraphy with technetium-99m mebrofenin. Morphine sulphate (0.04 mg/kg) was administered only if the gallbladder was not visualised after 1 h (16 patients). The final diagnosis was reached after clinical improvement, or upon the discovery of another aetiology for the symptoms presented, or on the basis of histopathology following cholecystectomy (when this was performed). We analysed the contribution of individual cholescintigraphic findings (I: non-visualisation of the gallbladder during the first 60 min of the examination; II: persistent non-visualisation of the gallbladder 30 min following morphine administration; III: non-visualisation of the small bowel for at least 90 min) and their various combinations. We obtained a sensitivity of 79% and a specificity rate 100% using the interpretative criteria "I and II or III". Excluding obstructive syndrome ("I and II"), the sensitivity and specificity figures were 70% and 100% respectively (28 patients). We had no false-positive results in our patient population. Cholescintigraphy was found to complement ultrasonography, which had either good sensitivity (93%) and poor specificity (17%), when at least two of the three major signs were present (sludge, thickened wall, gallbladder distension), or poor sensitivity (36%) and good specificity (89%) when all three signs were present. We conclude that cholescintigraphy is a useful tool for early diagnosis of AAC in critically ill patients, in whom ultrasonography alone does not provide enough information to permit a sufficiently early decision regarding the use of surgery.