Cyclin D1 is a G1 cyclin participating in the control of cell cycle progression through interaction with the retinoblastoma gene product (pRB). The overexpression of positive regulators (such as cyclin D1) has been reported in a variety of neoplasms, but their role in thyroid tumorigenesis is yet to be established. In our series of 54 thyroid carcinomas, cyclin D1 overexpression (detected by both immunohistochemistry and by Northern blotting) was correlated with prognostic variables, proliferative activity and pRB. Cyclin D1 overexpression was observed in 35% of thyroid carcinomas with a significantly higher expression of this cyclin in neoplastic tissues than in matched normal parenchyma. In well-differentiated carcinomas, the cyclin D1 mRNA overexpression was inversely correlated with nodal status (p = 0.03), while the protein product was higher in tumors from patients less than 40 than patients over 40 years of age. Inversely, there was no significant correlation with gender and tumor status, pRB and with proliferative activity.