Percutaneous balloon mitral commissurotomy was performed in 16 pregnant women aged 23 +/- 3 years (range, 16-39 years) who had severe mitral stenosis at pregnancies of mean gestational age 25 +/- 6 weeks. Ten patients were in New York Heart Association functional class III, and six patients were in functional class IV at the time of the procedure. All patients were symptomatic despite maximal medical therapy. The procedure was performed with the Inoue balloon. The mitral valve area increased from 0.9 +/- 0.3 to 1.8 +/- 0.3 cm2 (p < 0.05). The mitral valve pressure gradient decreased from 23 +/- 7 to 6 +/- 3 mm Hg (p < 0.05). The left atrial pressure decreased from 28 +/- 8 to 10 +/- 4 mm Hg (p < 0.05). The pulmonary artery pressure decreased from 59 +/- 18 to 33 +/- 12 mm Hg (p < 0.05). Fourteen patients continued their pregnancies to mean gestational age 37 +/- 2 weeks; all infants were healthy. Two patients had premature deliveries more than 1 month after the procedure due to obstetrical reasons. The two newborns died at day 2 of respiratory distress. Eleven women had vaginal deliveries and five had cesarean sections. All clinically improved to New York Heart Association functional class I or II. Excessive blood loss from the femoral puncture site that required transfusion occurred in one patient. Mitral regurgitation increased one degree in four patients, from 0 to 1+. Patients were observed until delivery. None had restenosis. The degree of mitral regurgitation remained unchanged. Percutaneous balloon mitral commissurotomy can be performed safely during pregnancy. It will effectively improve hemodynamics and symptoms in pregnant patients who have severe mitral stenosis and persistent congestive heart failure symptoms despite conventional medical treatment. There are no immediate detrimental effects of radiation on the fetus. Risks are lower than previously reported when surgical commissurotomy was performed.