PIVKA-II has been practically used as a tumor marker of hepatocellular carcinoma. On the other hand, increased serum PIVKA-II concentration was reported in a Japanese patient who had hyperthyroidism without liver diseases. To evaluate whether thyroid hormone is related with serum PIVKA-II, we examined serum PIVKA-II concentrations in patients with various thyroid diseases. Eight patients with Hashimoto disease, 24 patients with Graves' disease, and 8 healthy subjects were studied. There was no significant difference of serum PIVKA-II levels among the three groups. However, serum PIVKA-II concentrations(mean +/- SD mAU/ml) in hyperthyroidism(37 +/- 27) were significantly higher than those in hypothyroidism(16 +/- 9) and normal controls(12 +/- 4) (p < 0.05 and p < 0.01, respectively). When hyperthyroid patients were treated by antithyroid drug or isotope, serum PIVKA-II concentrations decreased in accordance with the decrease of serum FT4 concentrations. Our data indicate that serum PIVKA-II concentration was increased in patients with hyperthyroidism, but further in vivo studies are necessary to clarify the mechanism related to increased serum PIVKA-II by thyroid hormone.