Papillary thyroid cancer is the most common endocrine malignancy. Of all solid cancers presenting in adults, papillary thyroid cancer generally carries the best long-term prognosis. However, very little is understood about the molecular pathogenesis of this neoplasm. We recently hypothesized that increased nuclear levels of MDM2 protein might occur in well-differentiated papillary thyroid carcinomas (Gerasimov et al., Exp. Mol. Pathol. 62, 52-62, 1995). MDM2 is known to complex with and inactive the p53 tumor suppressor protein. Since p53 inactivation by gene mutation has an established role in the pathogenesis of undifferentiated (anaplastic) thyroid carcinoma, we reasoned that abrogation of p53 function by nuclear MDM2 protein accumulation might participate in the pathogenesis of certain well-differentiated thyroid cancers such as papillary cancer. In the present report we present the first direct evidence of MDM2 protein accumulation in the nuclei of papillary thyroid carcinoma cells in a subset of tumors. Using the IF-2 monoclonal antibody, which reacts specifically with human MDM2 protein, we studied 24 well-differentiated papillary thyroid carcinomas and 26 benign lesions (nodular goiters, adenomas, thyroiditis). Nuclear staining was quantitated using the CAS computerized image analysis system. We found positive nuclear MDM2 immunoreactivity in 8 (33%) of the carcinomas. In contrast, MDM2 staining was negative in all benign lesions (P = 0.001, two-tailed Fisher exact test). Normal thyroid tissue was also negative. These data suggest that nuclear accumulation of MDM2 protein might be implicated in the pathogenesis of a subset of papillary carcinomas. Further studies to investigate this possibility are warranted.