Monoclonal antibodies (mAbs) of murine origin, when labeled with radionuclides that emit gamma rays, target tumors, permitting detection of disease. Satumomab pendetide (Oncoscint CR/OV), a murine mAb, was recently approved by the FDA for the single-use detection, when labeled with indium-111, of extrahepatic intra-abdominal metastases from colorectal or ovarian cancer. Other radiolabeled mAbs are being explored for their diagnostic potential. The use of radiolabeled mAbs in the therapy of cancer is still far from routine, because the invariable development of antimouse antibodies following administration precludes repeat use, and because bone marrow toxicity limits the amount that can safely be given once. The development of nonimmunogenic antibody forms that will permit multiple administrations has renewed interest in radiolabeled mAbs. Also, better understanding of antigen heterogeneity and methods to upregulate antigen expression offer promise that radiolabeled mAbs may prove useful in treating established metastatic disease, as well as micrometastatic disease.