Bubble formation outside the digestive tract that correlated with small perforations was observed in two infants during water-soluble contrast material studies using a 50% solution of Gastrografin in water. The causes of extraluminal bubbles are discussed; in vitro experiments demonstrated the mechanism of formation and the radiographic characteristics of the various types of bubbles. Extraluminal bubbles appearing during a water-soluble contrast material examination actually represent contrast bubbles. The radiographic density of the bubble walls varies. When the bubbles are small and diluted contrast material is used, it is not possible to detect the presence of contrast material radiographically. Contrast material bubbles are likely to be observed only if a low surface tension contrast agent is used, such as Gastrografin full- or half-strength in water). Extraluminal contrast material bubble formation may identify and localize a small perforation of the gut when other findings are inconclusive.