Some patients who undergo free flap reconstruction may suffer pulmonary edema, requiring postoperative medical intervention including use of diuretic agents, such as furosemide. However, the effect of diuretic agents on a free flap remains unclear. We present a case of flap loss due to venous thrombosis following intravenous infusion of furosemide in a 36-year-old patient. The patient suffered from severe dyspnea and acute pulmonary edema caused by fluid overload during the perioperative period. Furosemide was administered intravenously for relieving dyspnea. After 4 hours, the flap showed signs of venous congestion. Surgical exploration revealed a venous thrombosis of the vascular pedicle with com-plete luminal occlusion. We would like to suggest that use of furosemide in patients underwent reconstruction with free flaps must be limited only to cases with evidence of acute respiratory failure. Even in these cases, intensive flap monitor-ing and preparation for early intervention on venous thrombosis will be required.