The ductus arteriosus in the fetus may contract after administration of nonsteroidal anti-inflammatory drugs such as indomethacin and aspirin. We report a similar effect observed after a 36-week pregnant women was given diclofenac against flank pains. The ductus of this fetus was vasoconstricted with evidence of right ventricular hypertension. It resolved after cessation of the drug. Diclofenac is a cyclo-oxygenase inhibitor and thus carries the pharmacodynamic properties of other nonsteroidal anti-inflammatory drugs with inhibition of prostaglandin synthesis, resulting in vasoconstriction of the ductus arteriosus. We suggest monitoring of the fetal ductus state and velocities by fetal echocardiography in women treated with diclofenac.