Aggrus (also known as T1α/podoplanin) is a membrane sialoglycoprotein whose function in tumors is unknown. We recently determined that Aggrus possessed the ability of inducing platelet aggregation and that its expression was frequently upregulated in colorectal tumors. Thus, Aggrus expression might be associated with tumor-induced platelet aggregation and tumor metastasis. Here we show, by means of cancer profiling array and real-time PCR, that aggrus mRNA expression is frequently upregulated in testicular germ cell tumors when compared with the surrounding normal tissue. Immunohistochemical staining revealed that Aggrus protein expression was detected in 10 of 11 seminomas (90.9%), but its expression was not observed in embryonal carcinomas (0/4; 0%). Specific markers for seminomas have not been reported, and Aggrus is a potential diagnostic marker for seminomas and may be associated with malignancies of the testis.