The effects of 5-fluorouracil (FU) and X-irradiation, separately and in combination, upon multiple metastatic tumors within individual patients are described. Fourteen cases satisfied the criteria for a controlled study. In 4, the effects of irradiation alone were greater than the combination, in 4 the effects were equal, and in 6 the effects of irradiation were less than those of combination therapy. This study failed to demonstrate a significant alteration in the tumor response by the addition of FU to X-irradiation. Four instances of unusual reactions in previously irradiated skin were noted following FU administration.