Since it is feasible to detect reverse T3 (rT3) in amniotic fluid, we investigated the possibility as to whether measurements of amniotic rT3 could be useful in diagnosing fetal hypothyroidism during pregnancy. In 55 amniotic fluid samples, obtained at different stages of pregnancy, we have documented increasing concentrations of this hormone. The results obtained are conflicting with previous reports. The reason for this discrepancy is not clear, however methodological differences in rT3 determination should be taken in account. The large scatter of rT3 values in amniotic fluid suggests that the diagnosis of neonatal hypothyroidism based on measurement of rT3 may require caution.