Ten pregnancies with 13 fetuses complicated by symptomatic polyhydramnios were treated with indomethacin. A baseline fetal echocardiogram was obtained before therapy, repeated 24 hours after the initiation of indomethacin and then performed weekly provided that the ductus arteriosus remained patent. The mean duration of indomethacin therapy was 28.3 +/- 18.9 days. The starting dose in all patients was 25 mg orally every six hours. The ductus arteriosus was noted to constrict in four patients, with the development of tricuspid regurgitation in one. One of the constrictions occurred after 23 days of therapy. In one patient with constriction a reduction of the indomethacin dosage was not associated with constriction, while in the other three, constriction was still evident at a reduced dosage. All constrictions and the tricuspid regurgitation resolved in utero within 24 hours after discontinuation of the indomethacin. We recommend careful monitoring of the fetal ductus arteriosus when treating symptomatic polyhydramnios with indomethacin.