Leu-M1 antigen is a monocyte/granulocyte-related marker known to be consistently expressed in the Reed-Sternberg cells of patients with Hodgkin's disease. Recently, however, the presence of Leu-M1 has also been noted in tumour cells of a variety of non-haematopoietic neoplasms, most of them adenocarcinomas. The biological significance of this aberrant reaction has not been clarified. We have been able to demonstrate marked epithelial Leu-M1 immunoreactivity (greater than 15% tumour cells positively stained) in 24 out of 76 (32%) papillary carcinomas of the thyroid gland (PC). This phenomenon was more frequently observed among PCs at an advanced stage of disease (pT4 vs. pT1-3 and M1 vs M0 p less than 0.05). The degree of epithelial Leu-M1 positivity was also shown to be significantly correlated to the clinical course of PC. Irrespective of other morphological and clinical features, death resulting from cancer occurred 17 times more frequently among PCs with marked Leu-M1 positivity (8/24) when compared with tumours with only slight or absent immunoreactivity (1/52) (p less than 0.00005). These findings suggest that Leu-M1 immunostaining provides significant prognostic information for patients with papillary carcinoma of the thyroid gland.