Immunoreactivity with monoclonal antibody CD15 (Leu-M1) was investigated in the primary tumours, the metastases and local recurrences of 47 cases of sporadic medullary carcinoma of the thyroid (MTC). Of these tumours, 36.5% showed a varying degree of CD15 immunostaining; in 7 carcinomas the CD15 immunoreactivity was found to be significant (greater than 15% tumour cells positively stained). Staining of the amyloid stroma was observed in 3 tumours. Significantly higher epithelial CD15 positivity was seen more frequently in the group with larger tumours (greater than 4 cm) and was found exclusively in the presence of lymph node metastases. No substantial difference in the percentage of immunostained cells was seen between primary tumours and metastatic or recurrent lesions, except for two cases that revealed a significant increase in the number of CD15-immunostained cells in metastatic and recurrent lesions. Five of 7 patients with recurrences showing significant CD15 immunostaining died of cancer, while in the absence of significant CD15 staining all patients with recurrences were still alive at the conclusion of the study. The prognostic value of CD15 immunoreactivity, found by univariate analysis, becomes weaker after adjustment for the size and stage of tumour. Particularly in patients with tumour recurrences CD15 immunostaining may be of clinical relevance for the selection of patients in whom a more radical surgical approach would be justified.