The distribution of 111In-labelled anti-carcinoembryonic antigen (CEA) monoclonal antibody fragments [F(Ab')2] was studied in five patients either with known inoperable medullary carcinoma of the thyroid (MCT) or evidence of recurrence/metastases based on elevated calcitonin (hCT) levels. All five cases had elevated serum CEA levels and positive immunohistochemical stains for both hCT and CEA prior to scintigraphy. In two patients with identified inoperable disease both planar and SPECT scans were positive. In the remaining three patients, where the recurrence/metastatic sites were unknown, SPECT images were positive in two. Of these, only one had positive planar images. These results indicate that 111In-labelled anti-CEA F(ab')2 scintigraphy, especially in conjunction with SPECT, is useful for the diagnostic evaluation of patients with MCT. The limiting factor of this technique is the high level of non-specific uptake, particularly in the liver, but improvements in the specificity of newer anti-CEA antibodies and the ability to label these with 99Tcm is addressing this problem.