Cyclin D1 plays a key role in the regulation of the G1/S transition through the cell cycle. Deregulation of cyclin D1, most often leading to overexpression of the gene, has been reported in many tumor types. It has been suggested that cyclin D1 overexpression could be an alternative mechanism for pRb inactivation. We have previously found Rb gene mutations in 55% of malignant thyroid tumors. In the present study, we examined the cyclin D1 gene expression and amplification in 24 tumor samples (two of them are benign goiters) randomly selected from the same series of thyroid tumors, to see whether cyclin D1 overexpression is present in those specimens without Rb gene mutations. We found a four- to fivefold increase in cyclin D1 expression in 7 of 22 thyroid carcinomas as compared with that in benign nodular goiters. Six of them were found in carcinomas without Rb gene mutations. Among the remaining 15 thyroid carcinoma samples, 11 were found previously to have Rb gene mutations. The association between increased cyclin D1 expression and absence of Rb mutation is statistically significant (p < 0.05). We found no evidence of the cyclin D1 gene amplification or rearrangement to account for such an increase in cyclin D1 expression. We conclude that cyclin D1 overexpression may be relevant to thyroid carcinogenesis. Two mechanisms may be involved in the inactivation of pRb: one is through Rb gene mutations, and the other is by cyclin D1 overexpression.