Gas within clefts of fissures in gallstones is not a very common finding, but when it occurs is, characteristic and indicates the presence of one or more calculi. It closely resembles the appearance of a 'Jack Stone' but has previously been termed the 'Mercedes Benz' sign. Only a few cases have previously been recognised in the U.K. Most reported cases have been associated with biliary colic or cholecystitis. Various theories have been put forward to explain the presence of gas, but the author believes that the gas is released from solution by negative pressure within cholesterol stones undergoing internal fissuring due to their crystalline structure, i.e. the gas is released from solution from the small amount of fluid trapped in the calculus, in the same way that gas may be 'pulled' out of solution in a joint, a degenerate intervertebral disc or the fibro-cartilage of the symphysis pubis.