The internal fluid pressure of 62 echinococcal hydatid cysts in various organs was measured intraoperatively in 36 patients. Growing hydatid cysts, whether intact univesicular, or previously ruptured multivesicular, demonstrated a fluid pressure distinctly higher than that of surrounding tissues, reflecting viability of the parasitic elements within the cyst. The highest pressure of 98 cm of water, was recorded for a univesicular cyst in the superior portion of the liver. Lung and intra-abdominal cysts registered lower internal pressure. Degenerating cysts showed little or no internal pressure. Relief of its internal fluid pressure seems a logical preliminary step in the surgical management of hydatid cysts