Considerable morbidity and mortality continue to attend the rupture of pancreatic pseudocysts. If the mechanism responsible for rupture were known, it is conceivable that patients with impending rupture could be identified and appropriate measures taken. In an effort to determine whether pseudocysts rupture from increased internal pressure, in vivo pressure measurements and stress calculations were performed in 10 intact pseudocysts. Next, fresh strips of pseudocyst wall tissue from seven patients were placed in a tensile testing machine, and ultimate strength (peak stress capacity) was determined. The average in vitro peak stress prior to yield and separation was 12.30 +/- 2.75 Kg-F/cm2. Both this value, and calculated internal pressures required to achieve this level of wall stress, are more than 100 times greater than the corresponding values ever measured in humans. In all likelihood, pseudocysts rupture from diminished wall thickness resulting from immaturity and/or enzymatic debridement.