Forty of 416 patients with familial adenomatous polyposis were noted to have intra-abdominal desmoid tumors, and a subgroup of 16 were treated with noncytotoxic drug therapy. Drugs used were sulindac (14 patients), sulindac plus tamoxifen (3 patients), indomethacin (4 patients), tamoxifen (4 patients), progesterone (DEPO-PROVERA; Upjohn Co., Kalamazoo, MI) (2 patients), and testolactone (1 patient). Therapy with these drugs for continuous periods of six months or more resulted in three complete and seven partial remissions. When treated patients were compared with untreated patients (n = 12), there were significant benefits for the treated group, both in reduction of desmoid size and in improvement of symptoms, despite the inherent selection bias against this. Sulindac was the only drug used in enough patients to permit independent evaluation of its effect, with one complete and seven partial reductions of tumor size. Some patients had a delayed response to sulindac, with tumor shrinkage occurring after an initial period of tumor enlargement. When using sulindac for the treatment of desmoid tumors, this phenomenon should be considered.