Ectopic production of isolated alpha-subunit of glycoprotein hormones was detected in a 32-year-old Japanese woman with metastatic lung tumor by a discordance in the immunologic pregnancy tests for native human chorionic gonadotropin (hCG) assays. Quantitative discrepancy of titrations among different types of pregnancy tests, ie, competitive (hemagglutination or latexagglutination inhibition) and noncompetitive (hemagglutination or latexagglutination) reactions, was also confirmed experimentally using a highly purified ectopic alpha-subunit from the patient's urine. Thus the discrepancy between the 2 types of assay systems was due, at least partly, to a cross-reaction of high concentrations (20,000 ng/ml) of ectopically secreted free alpha-subunit of glycoprotein hormones.