Midkine (MK) is a novel heparin-binding growth factor whose gene has been identified in embryonal carcinoma cells in early stages of retinoic acid-induced differentiation. We immunohistochemically examined 90 thyroid papillary carcinomas (85 invasive type and five encapsulated type), using a rat IgG2a monoclonal antibody against the carboxyl terminal region of human MK in archival paraffin sections. The thyroid tumors exhibited an intense reaction in the cytoplasm. Most of the papillary carcinomas (77/90), had tumor cells that expressed MK. These were classified into the following two types: invasive type (76/85) and encapsulated type (1/5). Notably, the intensity of MK was stronger at the invading border area of the tumors than in the center. In tissues adjacent to the cancer tissues, normal follicular epithelial cells expressed MK very faintly or not at all. The in situ hybridization analysis revealed that the signals of MK transcripts were found in the cytoplasm of the cancer cells. In the noncancerous follicular epithelial cells adjacent to neoplasm the signals of MK transcripts were detected very weakly or not at all. The distribution and localization of the MK-transcript signals determined by in situ hybridization analysis were similar to those obtained by immunohistochemical analysis. We conclude that thyroid papillary carcinoma strongly expresses MK protein and messenger RNA, and that this overexpression may relate to the development and invasion of these carcinomas.