Overexpression of cyclin D1 occurs in several malignancies, often due to gene amplification, and this has been associated with aggressive tumor behavior, a higher incidence of lymph node metastases, and a poorer prognosis. The role of cyclin D1 in the pathogenesis of thyroid malignancy is unknown; however, cyclin D1 expression has been reported to occur in a proportion of well differentiated thyroid carcinomas. Micropapillary carcinomas of the thyroid are common incidental findings that almost always behave in an indolent manner and remain quiescent. However, rare microcarcinomas behave aggressively and metastasize early, giving rise to clinically significant disease. We hypothesized that cyclin D1 might play a role in the aggressive behavior of metastasizing papillary microcarcinomas. We reviewed the histopathology reports of 2,000 patients who underwent thyroid surgery at our institution between 1995-1999 and identified 22 patients who presented with gross regional metastases from a primary papillary microcarcinoma. These patients formed the index cohort for this analysis. As controls, we selected 34 patients with nonmetastasizing microcarcinomas. We studied these tumors for immunoreactivity to cyclin D1 on immunohistochemistry and analyzed 13 tumors that diffusely expressed cyclin D1 for gene amplification by differential PCR. Twenty of the 22 (90.9%) metastasizing papillary microcarcinomas expressed cyclin D1, compared with 3 of the 34 (8.8%) nonmetastasizing papillary microcarcinomas (P < 0.001). However, of the 13 tumors that showed diffuse immunoreactivity for cyclin D1 on immunohistochemistry, none showed amplification of the cyclin D1 gene on differential PCR. We conclude that cyclin D1 is significantly overexpressed in metastasizing papillary microcarcinomas of the thyroid. This is likely due to mechanisms other than gene amplification. Cyclin D1 immunohistochemistry may be a valuable tool in predicting metastatic potential in papillary microcarcinomas.