Radioimmunodetection of colorectal cancer was evaluated in 51 patients by injecting 131I-labeled goat antibody immunoglobulin G against carcinoembryonic antigen and performing total-body photoscans with a gamma scintillation camera 24 and 48 h later. The scintigrams were then processed by computer to subtract the images of the 99mTc-serum albumin and pertechnetate administered, which reflect background and nontarget radioactivity, from the 131I-antibody scans. The results indicate that radioimmunodetection is a safe and a potentially clinically useful cancer detection method, which in this study demonstrated primary colorectal carcinomas in 10 of 12 (83%) of the patients evaluated preoperatively and between 87% (46 of 53) and 92% (49 of 53) of known metastatic tumor sites. Thus, the method's overall sensitivity (true-positive rate) was 86%-91% on a tumor-site basis. A false-negative rate of between 9% and 14% and a false-positive rate of less than 4% were found. In 11 of the 51 patients evaluated, tumor sites were detected that were not found by other clinical methods of cancer detection. These sites of tumor were then confirmed later, as much as 40 wk after radioimmunodetection was performed. It is concluded that in colorectal cancer patients, the current method of carcinoembryonic antigen radioimmunodetection can (a) contribute to the preoperative clinical staging of the patients, (b) assist in the postoperative evaluation of tumor recurrence or spread, (c) complement other methods used to assess tumor response to therapy, (d) support the indication of a rising carcinoembryonic antigen titer (when other methods cannot detect tumor) for second-look surgery, and (e) confirm the findings of other detection measures that are less tumor-specific.