Several prognostic factors have been proposed to identify the patients at risk to develop metastases in differentiated thyroid carcinoma. Reduced nm23-H1 expression (a metastatic suppressor gene) has been correlated with high tumor metastatic potential in various human carcinomas, but the results obtained in differentiated thyroid carcinoma remain controversial. To elucidate the usefulness of nm23-H1 as a differentiated thyroid carcinoma prognosis factor, we evaluate the relationship between nm23-H1 immunoreactivity as well as both clinical status and patient outcome. For this purpose, thyroid resected specimens obtained from 94 differentiated thyroid carcinoma consecutive patients (64 papillary and 30 follicular) with at least 5 yr of follow-up were stained using monoclonal antibody to nm23-H1. We did not observe any relationship between nm23-H1 immunoreactivity and age, gender, initial differentiated thyroid carcinoma stage, local recurrence, or distant metastases in patients with papillary carcinoma. However, in patients with follicular carcinoma, a significant inverse association between metastatic disease and the expression of nm23-H1 product was obtained (P < 0.05). In addition, significant differences were found in the survival curves according to nm23-H1 immunoreactivity (log-rank P < 0.01). Finally, nm-23-H1 immunoreactivity was more specific but less sensitive than AMES score to predict metastases. In conclusion, our results suggest that nm23-H1 immunostaining could be added to the classic prognostic factors currently used to predict the outcome of patients with follicular thyroid carcinoma.