Serious complications are likely to accompany the treatment of giant ovarian tumors, and resection with or without preoperative drainage has been previously reported. Here, we report the case of a 27-year-old Japanese woman with a significant weight gain of 50 kg, who was referred to the Kumamoto University Hospital because of gait impairment and dyspnea. Imaging tests revealed an ovarian tumor, 37 cm in diameter, with two solid components. The patient's condition improved after the removal of 31.5 l tumor fluid by using a suprapubic urinary catheter for 3 days. The tumor was subsequently resected without complications, and was diagnosed as a left mucinous ovarian tumor with malignant components, weighing 37 kg (81.5 lb). The patient was discharged after her anasarca improved, and her body weight decreased from 100 to 50 kg with accompanying considerable urination within two weeks. She was in good condition with no evidence of recurrence at 15 months after surgery. Tumor resection after preoperative drainage was effective in the management of a patient with dyspnea induced by a giant ovarian tumor. We suggest the use of a suprapubic urinary catheter for preoperative drainage because of its ease of use in preventing fluid leakage from the possibly malignant tumor.