The management of intra-abdominal desmoid tumors in patients with familial adenomatous polyposis (FAP) is very difficult. Non-steroidal anti-inflammatory drugs (NSAIDs), anti-estrogenic agents, and steroids are most commonly used, because surgical removal of these tumors may result in severe morbidity, with local recurrence being common. We report a patient with FAP and intra-abdominal desmoid tumors that regressed markedly after prednisolone therapy. The patient, a 38-year-old woman, had undergone total colectomy and ileorectal anastomosis with a diagnosis of FAP with colon cancer. Approximately 17 months after the surgery, she noticed an elastic firm lump in the abdominal wall. She also experienced lower abdominal distension. Computed tomography (CT) of the lower abdomen showed an invasive heterogenous low-density mass occupying the intra-abdominal space. She was treated with sulindac, NSAID, at 300 mg/day, the diagnosis being intra-abdominal desmoid tumors. She exhibited an intestinal obstruction about 9 months after the initiation of sulindac therapy. We changed the treatment and began prednisolone (initial dose, 40 mg/day). This treatment was continued for two years; subsequently, the lesions regressed markedly. She is currently well, more than 3 years after the withdrawal of prednisolone.