Erythrocyte zinc concentration and plasma thyroid hormones were measured in 32 patients admitted to hospital because of hyperemesis gravidarum. At presentation, 8 patients had elevated total and free thyroxine concentrations. The initial erythrocyte zinc concentration in these 8 patients was not different from the others, but in 5 of them, a transient drop in the erythrocyte zinc concentration was observed 2–12 weeks later, and which returned to normal towards the end of pregnancy. The lowest erythrocyte zinc concentration in the hyperthyroxinaemic group (171.0 ± 31.7μmol/l of RBC) was significantly lower than that of the normothyroxinaemic group (214.2 ± 33.9 μmol/l of RBC) (p < 0.01). This indicates that hyperthyroxinaemia in patients with hyperemesis gravidarum represents true hyperthyroidism, but is different from classical thyrotoxicosis in that the elevation of thyroid hormones is transient.