OBJECTIVE Previous research has identified an association between idiopathic edema and obstructive sleep apnea (OSA) in women, but a causal relationship between OSA and edema has not been established. This study was undertaken to determine whether nasal continuous positive airway pressure (CPAP) lessens edema in patients with idiopathic edema and OSA. METHODS This was a case-series study that enrolled eight ambulatory, obese, adults with bilateral, pitting pre-tibial edema and OSA from an inner city family practice and a suburban family practice from July 1995 until March 2003. Enrollees underwent subjective and physical examination assessment of changes in edema after initiation of nasal CPAP. RESULTS All but one of the subjects had severe OSA. The edema was typically 1+ to 2+, and the duration of the edema ranged from 6 months to more than 20 years. Seven of the eight subjects experienced a reduction in the amount of edema following nasal CPAP (P=0.04). CONCLUSIONS In subjects with OSA and idiopathic edema, nasal CPAP reduces the amount of edema. If valid, these results indicate that OSA can cause edema.