Cyclin-dependent kinase inhibitors, including the protein product of the p27/kip1 gene, play an important role in cell-cycle regulation. Loss of p27 expression was reported in a number of neoplasms and shown to be an independent prognostic factor in colorectal, lung, and breast carcinoma By immunohistochemical analysis, we investigated p27/kip1 expression, using a polyclonal antibody, in a series of 87 benign and malignant thyroid neoplasms. We correlated its expression with the Ki-67 labeling index and other prognostic factors. All of the thyroid neoplasms examined exhibited significantly lower p27 expression than did normal thyroid tissue (P < .001). Poorly differentiated carcinomas had the lowest p27 staining frequency of all carcinomas examined. p27 staining frequency of the papillary carcinomas was significantly lower than that of the follicular carcinomas (P < .001). This difference could not be attributed solely to the inverse correlation between the staining patterns of p27 and Ki-67, which was reported for other neoplasms, because there was no significant difference between the Ki-67 labeling indices of these two groups. The follicular variant of papillary carcinoma had a significantly higher p27 staining frequency (P = .05) than did classical papillary carcinoma. We saw no significant difference in the p27 staining frequencies between minimally and widely invasive follicular carcinomas nor between localized and nonlocalized papillary carcinoma. In summary, the p27 immunostaining pattern of thyroid neoplasms is related to neoplastic transformation and varies according to tumor phenotype. It seems, however, to have limited routine diagnostic or prognostic significance in thyroid neoplasia.