Accumulation of fluid in the lower legs occurs due to acute decompensated heart failure, venous deficiency, lymphedema, and a number of other medical conditions. An instrumented sock using an elasticity sensor is developed for the purpose of monitoring lower leg fluid status. The design and sensing principles of the sock are introduced. Two generations of prototype elasticity sensors have been constructed to verify the sensing principles. Their performances are analyzed and compared. Both in vivo and in vitro tests using the fabricated sensor prototypes show promising results.