This study was designed to examine whether measurement of serum CA19-9 was useful in testicular germ cell tumor patients. We analyzed the clinical courses of 55 testicular germ cell tumor cases diagnosed after high orchiectomy. The patients in this study consisted of 33 seminomas and 22 non-seminomatous germ cell tumors (NSGCT), and their mean age was 32.7 +/- 12.7 years (mean +/- SD). The mean follow-up period after the operation was 33.7 months. The positive rate of the pre-treatment serum CA19-9 level was 16.4% (3.0% in seminomas versus 36.4% in NSGCT, p = 0.0017). The pre-treatment serum CA19-9 levels in NSGCT patients were significantly higher than those in seminoma patients (46.6 +/- 50.0 U/ml versus 10.6 +/- 9.6 U/ml, p = 0.0008). We divided the patients into two groups according to the detailed histological types, and found that the serum CA19-9 levels in the patients with embryonal carcinoma (EC) were significantly higher than in those without EC (p = 0.0160), and the levels in those with yolk sac tumor (YS) were higher than in those without YS (p = 0.0099). Moreover, the levels in those with either EC or YS were significantly higher than in those with neither EC nor YS (p = 0.0004). In 9 patients with a high serum pre-treatment CA19-9 level, the serum CA19-9 level was useful as a monitoring marker through the treatment or tumor progression. On the other hand, the pre-treatment serum CA19-9 level did not correlate with the clinical stage or prognosis. In conclusion, the phenomenon that the serum levels of CA19-9 increase in testicular germ cell tumor patients is not extremely rare, and in NSGCT, especially in EC or YS, the serum CA19-9 can be a useful tumor marker.