In about 95% of patients with acute cholecystitis the cystic duct is obstructed by a gall stone. The imprisoned bile salts have a toxic action on the gall bladder wall. Acute cholecystitis is liable to be confused with other causes of sudden pain and tenderness in the right hypochondrium. Below the diaphragm, acute retrocecal appendicitis, intestinal obstruction, a perforated peptic ulcer or acute pancreatitis may be confusing factors; however, the gall bladder remains shrunken, fibrotic, full of stones and nonfunctioning. Recurrent acute cholecystitis may follow, but there may be surprisingly long clinically silent periods. The treatment of choice is elective cholecystectomy. General measures include bed rest, intravenous fluids, a light diet and relief of pain with pethidine and buscopan. Antibiotics are given to treat septicemia and prevent peritonitis and empyema. During the first 24 h., 30% of the gall bladder cultures are positive. This rises to 80% after 72 h. Common infecting organisms are Escherichia coli, Streptococcus faecalis and Klebsiella, often in combination. Anaerobes are present, if sought, and are usually found with aerobes. They include Bacteroides and Clostridia. Antibiotic(s) should have a spectrum to cover the colonic type micro-organisms which are usually found with infection of the biliary tree. The choice depends upon the clinical picture. A broad-spectrum penicillin or a cephalosporin is usually adequate for the stable patient with pain and mild fever. The severely septicemic patient is better treated with a combination of ureidopenicillin (mezlocillin or piperacillin) and metronidazole.