Follow-Up: Hello Jean. The use of CPAP can decrease the amount of urine that you produce with a consequent increase in water retention and body weight. For the first two weeks of CPAP therapy I suggest that you weigh yourself every day when you wake up in the morning. If you notice a sudden increase in your body weigh (more than 3-4 pounds in one day), please contact your physician. This is especially important if you have a heart condition. He may prescribe a "water pill" or diuretic for you. I believe there are several reasons for this phenomena in CPAP users, including changes in antidiutetic hormone secretion. But the most likely explanation is the hemodynamic (heart pumping) changes that take place with increased airway pressure. Consider for a moment that your body in an enclosed fluid system with the heart acting as a pump. The left side of the heart pumps blood (fluid) to the entire body and back to the right side of the heart. The right side of the heart pumps blood through the lungs to the left side of the heart. Now when you are just falling to sleep and, possibly, at other times of the night, your airway is open and the pressure that normally splints the collapsible airway open is transmitted to the lower airway and lungs. This causes the pressure in the lungs to increase and they hyperinflate. This then causes a decrease in blood volume that would normally return to the right side of the heart to "pool" in the lower extremities (the feet and ankles) and cause swelling, weight gain, and decreased urination. This is usually a temporary situation as your body adjusts to the increased airway pressure while you sleep. If it persists, contact your physician. Also, if you experience increased shortness-of-breath or are more tired than usual, contact your physician right away. I hope that this information has helped. Please contact me if you have any further questions please feel free to contact me. Good luck and persevere.