AbstractPlasma carcinoembryonic antigen (CEA) assay was done in 30 patients with osteosarcoma. CEA was found positive (> 2.5 ng/ml) in 17 of 21 patients who had active evidence of disease and negative (< 2.5 ng/ml) in all 9 patients who were in complete remission resulting from previous amputation or chemotherapy. Serial CEA determinations demonstrated a fall to normal in 7 of 9 patients following successful surgery or chemotherapy and a rise and fall (fluctuation) of levels in 8 patients who had tumor progression while on chemotherapy. Clinical recurrence of disease in two instances preceded or coincided with CEA elevation. The CEA assay in osteosarcoma although nonspecific could be used as an inportant adjunct to experienced clinical judgment, periodic x‐ray examination, and laboratory study to prognosticate the course of osteosarcoma during therapy. The interpretation of a rising or falling CEA titer alone, however, must be made with caution.