With the introduction of immunoscintigraphy (IS) with 99mTc-labelled anti-CEA monoclonal antibodies (MoAb), a clinical relevant method in nuclear medicine can be expected in the diagnosis and follow up of colorectal cancers. We performed IS (whole body, planar and SPECT) with a 99mTc-labelled intact anti-CEA MoAb (BW 431/26) in 18 patients with primary colorectal carcinoma, metastases or suspicious recurrences from colorectal carcinoma. The results of anti-CEA IS, serum CEA and Ca 19-9 levels were evaluated. Immunoscintigraphy yielded an overall sensitivity of 70.0%, 37.5% for primary tumors, 75.0% for recurrences and 100% for distant metastases. Serum CEA levels were elevated in 10 out of 18 patients (sensitivity 55.5%) and Ca 19-9 were elevated in eight out of 18 patients (sensitivity 44.4%). In the group of patients with metastases, CEA had a sensitivity of 100% and Ca 19-9, of 83.3%. From this prospective study, we can conclude that IS with 99mTC-BW 431/26 is a reliable tool in the post-operative follow-up study of patients with colorectal carcinoma, namely in the detection of distant metastases.