A 32-year-old male consulted a hospital with a complaint of left scrotal swelling. Serum hCGbeta and LDH levels were elevated and computed tomography demonstrated a suspicious small lymphadenopathy in the paraaortic region. Left inguinal orchiectomy was performed. Histological examination demonstrated seminoma. We diagnosed the disease as clinical stage 2A and the patient received 2 courses of chemotherapy with cisplatin, etoposide and bleomycin. After this therapy, the small lymphadenopathy in the paraaortic region did not decrease in size. We considered this lesion a vessel or connective tissue. Although he achieved clinical complete remission and serum LDH level was normalized, the serum hCGbeta level remained low level positive. Urinary hCGbeta level after chemotherapy was below the threshold of detectability. We measured the serum from this patient as well as control sera by two- and four-fold dilution with a diluent comprised of mouse serum as a heterophilic antibody-blocking agent. The serum hCGbeta level of this patient was obviously decreased; in contrast, control sera were decreased in parallel. The serum hCGbeta level of this patient remained low level positive without recurrence for 8 months after chemotherapy. These results strongly suggested that low level of positivity for serum hCGbea in this case was a false positive finding. We consider the measurement of urinary hCGbeta and dilution measurement using a heterophilic antibody-blocking agent to be useful methods of distinguishing false positive findings for serum hCGbeta.