With the introduction of 99mTc-labeled monoclonal antibodies against carcinoembryonic antigen (CEA) a clinically relevant extension can be expected in the diagnosis of colorectal tumors by immunoscintigraphy (IS). This study comprises a total of 49 patients (primary colorectal tumors, occult neoplasms, and suspicious recurrences), in whom IS with 99mTc monoclonal antibody (MAb) BW 431/26 was performed. After injection of 1100 MBq 99mTc MAb BW 431/26 a whole-body scan was performed in anterior and posterior projection 5 1/2 h later, and SPECT of the abdominal region was done after 6 and 24 h. In the course of primary tumor identification (n = 20) all coloscopically diagnosed and operatively verified carcinomas were confirmed and correctly localized by IS (n = 11). In three patients with positive IS and suspicious coloscopic findings surgery was refused by patients and relatives. In five cases IS was true negative and in 1 case false positive. In the diagnosis of recurrences (n = 29) IS showed an uptake in computer-tomographically and coloscopically suspicious areas in 17 cases. In 12 cases IS was rated negative (11 true-negative findings in scar and granulation tissue, 1 false-negative finding in para-aortal lymph nodes). Elevated serum CEA levels were found only in 17 of 31 patients with true-positive IS. In postoperative cancer care IS with 99mTc-labeled anti-CEA antibody plays a preeminent role in the exclusion or identification of colorectal recurrences.