The effect of treatment with the nonsteroid anti-inflammatory drug indomethacin, was investigated in 250 3-month-old C57Bl female mice following induction of esophageal tumors with diethylnitrosamine (DEN). Already 3 months after DEN treatment, many esophageal tumors were committed to occur: despite discontinued DEN treatment, the number of tumors had increased significantly at 6 or 9 months of observation. Following 3 and 4 months of DEN administration, the development of many already committed esophageal tumors was significantly abrogated by treating the mice with indomethacin.