A 31-year-old man with a left testicular mass was treated with left high orchiectomy. Histological and immunohistochemical findings indicated a diagnosis ofseminoma. Computed tomography (CT) showed multiple enlarged lymph nodes. Serum biochemical examination showed an elevated serum lactate dehydrogenase (LDH) level. He was diagnosed with seminoma grade cT1N3M1aS2, stage IIIB. We administered four courses of bleomycin, etoposide and cisplatin (BEP) chemotherapy. Subsequent CT showed one residual lymph node measuring ＞3 cm in diameter. His serum LDH level was transiently elevated at the end ofeach course ofchemotherapy. Two additional courses ofetoposide and cisplatin (EP) chemotherapy were administered because it was suspected that the elevated serum LDH levels indicated residual tumor. Another possible cause of the elevated serum LDH levels was an adverse effect of granulocyte colony stimulating factor (G-CSF) therapy. The first course of EP chemotherapy did not include G-CSF administration, and there was no subsequent increase in his serum LDH level. The second course included G-CSF administration, and his serum LDH level increased simultaneously with the increase in white blood cell count. We concluded that the transient elevations in serum LDH level were an adverse effect of G-CSF therapy rather than an indication of residual tumor. His serum LDH level did not increase significantly after subsequent courses of chemotherapy. Eight weeks after the end of chemotherapy, positron emission tomography-CT showed no evidence ofresidual or recurrent tumor.