The action of piroxicam, an inhibitor of prostaglandin synthesis, was evaluated in 31 cases of measurable pulmonary metastases of several primary cancers. In each case, the spontaneous doubling time was measured prior to and after a minimum of 3 months of treatment with piroxicam as a single drug, 20 mg daily po. The doubling time of metastatic lung nodules was significantly increased in 18/31 cases (58%) of which 10 cases had their spontaneous doubling time multiplied by two. Minor regressions were observed in 5/31 cases (16%) and one case of metastatic fibrosarcoma had a complete radiologic response. No effect was observed in 8/31 cases (26%). Prostaglandin synthesis inhibitors may be useful for treating metastatic cancer. Possible mechanisms of action are discussed.