Using a highly sensitive immunoradiometric assay kit for human TSH, we measured TSH concentrations in unconcentrated amniotic fluids in normal pregnancies and those complicated for example by maternal hyper- and hypothyroidism, and compared them with those in maternal and cord sera. In normal pregnancies the mean concentration of TSH in amniotic fluid samples was 0.129 microU/ml, ranging from 0.065 to 0.278 microU/ml. In patients with premature delivery, the amniotic fluid TSH concentration was higher at 0.218 microU/ml. In four patients with abnormal thyroid function, TSH in amniotic fluid changed in parallel to that in cord serum, and there was a significant positive correlation between the two. No such correlation was observed between the concentrations of TSH in amniotic fluid and maternal serum. These results suggest that TSH in human amniotic fluid reflects fetal rather than maternal thyroid function and that the determination of TSH levels in amniotic fluid is useful in the diagnosis of abnormal thyroid function in fetuses.